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What Is Homeostasis in Biology? Definition and Examples

Homeostasis is a fundamental concept in biology that refers to the self-regulating process by which biological systems maintain stability while adjusting to changing conditions. This stability, or equilibrium, is essential for organisms to function effectively and efficiently.

Simple Definition of Homeostasis

Homeostasis is the ability of an organism to maintain a stable internal environment despite changes in external conditions. This process involves various biological mechanisms that detect changes, trigger responses, and restore balance. Examples of things that homeostasis controls include body temperature, chemical energy, pH levels, oxygen levels, blood pressure, and blood sugar.

Origin and History of Discovery

The word “homeostasis” originates from the Greek words ‘homeo,’ meaning similar, and ‘stasis,’ meaning standing still. Walter Cannon, an American physiologist, coined the term in the early 20th century. He built upon the work of Claude Bernard, a French physiologist who first recognized the concept of an internal milieu in the mid-19th century.

Components of Homeostasis

Homeostasis involves three primary components:

• Receptors : These are structures that detect changes in the environment (internal or external) and send this information to the control center.
• Control Center : Usually the brain or endocrine system, it processes the information and determines the appropriate response.
• Effectors : These are organs or cells that enact the response determined by the control center, thereby restoring balance.

A classic example of homeostasis involving receptors, control center, and effectors is the regulation of blood glucose levels in the human body. This process maintains the energy supply to cells and is tightly controlled.

1. Receptors: Detecting Blood Glucose Levels

In this context, receptors are specialized cells in the pancreas that monitor glucose levels in the blood. These cells are known as pancreatic beta cells. When blood glucose levels rise (such as after eating), these cells detect the increased glucose.

2. Control Center: Pancreas as the Decision-Maker

Upon detecting high glucose levels, the beta cells of the pancreas serve as the control center. They assess the information from the receptors and determine the necessary response to restore glucose levels to a normal range. The pancreas then synthesizes and releases the hormone insulin into the bloodstream.

3. Effectors: Actions to Lower Blood Glucose

The effectors in this process are primarily the liver and muscle cells, which respond to the insulin released by the pancreas. Insulin signals these cells to increase the uptake of glucose from the blood. Muscle cells use glucose for energy, especially during physical activity. The liver converts excess glucose into glycogen for storage, effectively lowering the blood glucose level and restoring equilibrium.

Positive and Negative Feedback in Homeostasis

Feedback mechanisms maintain the stability in the body’s internal environment. There are two types of regulatory mechanisms: negative feedback and positive feedback.

Negative Feedback

Negative feedback is the most common feedback mechanism in homeostasis. It counteracts or negates a change, bringing the system back to its set point or equilibrium. When a deviation from a set point is detected, negative feedback mechanisms initiate responses that reverse the change and restore balance. Key characteristics include:

• Self-limiting : Once the desired level is reached, the response diminishes or stops.
• Examples : Body temperature regulation (sweating to cool down when hot, shivering to warm up when cold), blood glucose regulation (insulin and glucagon balancing glucose levels).

Positive Feedback

Positive feedback is less common in homeostasis. This type of feedback amplifies a change or deviation, pushing the system further away from its set point. This mechanism is useful in situations where a rapid, decisive change is beneficial. Characteristics of positive feedback include:

• Self-amplifying : The response enhances the change, leading to an even greater response.
• Controlled and Temporary : Usually, positive feedback is part of a larger negative feedback system and is short-lived.
• Examples : Blood clotting (where each step in the clotting process triggers the next), the release of oxytocin during childbirth to intensify labor contractions.

Both negative and positive feedback mechanisms are crucial for maintaining homeostasis, though they operate differently. Negative feedback maintains stability and balance, while positive feedback aids specific, often critical, functions that require a rapid or substantial change.

More Examples of Homeostasis

Examples in humans.

• Water Balance : The body regulates water balance through mechanisms like thirst, urine production, and sweating to prevent dehydration or overhydration.
• Temperature Regulation : The body maintains an internal temperature around 37°C. When body temperature rises, mechanisms like sweating and increased blood flow to the skin help cool the body.
• Blood pH Regulation : The body maintains the pH of blood (around 7.35-7.45) through the respiratory system (by altering breathing rates) and kidneys (by excreting H + ions).
• Calcium Levels : Regulation of calcium levels in the blood is controlled by hormones like parathyroid hormone and calcitonin, affecting bone, kidney, and intestinal activities.
• Oxygen and Carbon Dioxide Levels : The respiratory system maintains a balance in oxygen and carbon dioxide levels in the blood through changes in breathing rate and depth.
• Electrolyte Balance : Sodium, potassium, and chloride ions are regulated to maintain nerve and muscle function, fluid balance, and acid-base balance.

Examples in Other Organisms

• Thermoregulation in Birds and Mammals : Many birds and mammals maintain a constant body temperature through mechanisms like shivering, sweating, panting, and adjusting their metabolic rate.
• Osmoregulation in Fish : Fish maintain the balance of water and salts in their bodies, despite the salt concentration in their environment. Freshwater fish actively excrete water and retain salts, while marine fish do the opposite.
• Stomatal Regulation in Plants : Plants open and close stomata to balance CO 2 intake for photosynthesis with water loss through transpiration.
• pH Regulation in Marine Life : Marine organisms like corals and mollusks regulate the pH within their cells and bodily fluids to counteract the acidification of ocean water.
• Hibernation in Bears and Other Animals : Hibernation is a form of long-term homeostasis where animals slow their metabolism, reduce body temperature, and conserve energy during scarce food availability in winter.

Microbial Homeostasis

Even microorganisms like bacteria exhibit homeostasis. For instance, they regulate their internal pH, ion concentrations, and respond to osmotic stress by synthesizing or importing compatible solutes.

Importance of Homeostasis

Homeostasis is crucial for the survival of organisms. It ensures optimal operating conditions for cells and organs, facilitates physiological processes, and maintains a balance despite environmental changes. Disruption in homeostasis often lead to diseases or disorders, reflecting its importance in health and disease.

• Aronoff, Stephen L.; Berkowitz, Kathy; et al. (2004). “Glucose Metabolism and Regulation: Beyond Insulin and Glucagon”. Diabetes Spectrum . 17 (3): 183–190. doi: 10.2337/diaspect.17.3.183
• Betts, J. Gordon; Desaix, P.; et al. (2013) Anatomy and Physiology (1st ed.). OpenStax. ISBN: 9781947172043.
• Boron, W.F.; Boulpaep, E.L. (2009). Medical Physiology: A Cellular and Molecular Approach (2nd International ed.). Philadelphia, PA: Saunders/Elsevier. ISBN 9781416031154.
• Kalaany, N.Y.; Mangelsdorf, D.J. (2006). “LXRS and FXR: the yin and yang of cholesterol and fat metabolism”. Annual Review of Physiology . 68: 159–91. doi: 10.1146/annurev.physiol.68.033104.152158
• Marieb, E.N.; Hoehn, K.N. (2009). Essentials of Human Anatomy & Physiology (9th ed.). San Francisco: Pearson/Benjamin Cummings. ISBN 978-0321513427.

Related Posts

1.5 Homeostasis

Learning objectives.

By the end of this section, you will be able to:

• Discuss the role of homeostasis in healthy functioning
• Contrast negative and positive feedback, giving one physiologic example of each mechanism

Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A set point is the physiological value around which the normal range fluctuates. A normal range is the restricted set of values that is optimally healthful and stable. For example, the set point for normal human body temperature is approximately 37°C (98.6°F) Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback. Negative feedback is a mechanism that reverses a deviation from the set point. Therefore, negative feedback maintains body parameters within their normal range. The maintenance of homeostasis by negative feedback goes on throughout the body at all times, and an understanding of negative feedback is thus fundamental to an understanding of human physiology.

Negative Feedback

A negative feedback system has three basic components ( Figure 1.10 a ). A sensor , also referred to a receptor, is a component of a feedback system that monitors a physiological value. This value is reported to the control center. The control center is the component in a feedback system that compares the value to the normal range. If the value deviates too much from the set point, then the control center activates an effector. An effector is the component in a feedback system that causes a change to reverse the situation and return the value to the normal range.

In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (that is, beyond homeostasis). This stimulus is “heard” by a specific sensor. For example, in the control of blood glucose, specific endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream. These pancreatic beta cells respond to the increased level of blood glucose by releasing the hormone insulin into the bloodstream. The insulin signals skeletal muscle fibers, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream. As glucose concentration in the bloodstream drops, the decrease in concentration—the actual negative feedback—is detected by pancreatic alpha cells, and insulin release stops. This prevents blood sugar levels from continuing to drop below the normal range.

Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain ( Figure 1.10 b ). When the brain’s temperature regulation center receives data from the sensors indicating that the body’s temperature exceeds its normal range, it stimulates a cluster of brain cells referred to as the “heat-loss center.” This stimulation has three major effects:

• Blood vessels in the skin begin to dilate allowing more blood from the body core to flow to the surface of the skin allowing the heat to radiate into the environment.
• As blood flow to the skin increases, sweat glands are activated to increase their output. As the sweat evaporates from the skin surface into the surrounding air, it takes heat with it.
• The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. This further increases heat loss from the lungs.

In contrast, activation of the brain’s heat-gain center by exposure to cold reduces blood flow to the skin, and blood returning from the limbs is diverted into a network of deep veins. This arrangement traps heat closer to the body core and restricts heat loss. If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and producing shivering. The muscle contractions of shivering release heat while using up ATP. The brain triggers the thyroid gland in the endocrine system to release thyroid hormone, which increases metabolic activity and heat production in cells throughout the body. The brain also signals the adrenal glands to release epinephrine (adrenaline), a hormone that causes the breakdown of glycogen into glucose, which can be used as an energy source. The breakdown of glycogen into glucose also results in increased metabolism and heat production.

Interactive Link

Water concentration in the body is critical for proper functioning. A person’s body retains very tight control on water levels without conscious control by the person. Watch this video to learn more about water concentration in the body. Which organ has primary control over the amount of water in the body?

Positive Feedback

Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite end point. Childbirth and the body’s response to blood loss are two examples of positive feedback loops that are normal but are activated only when needed.

Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired. Enormous changes in a person’s body are required to expel the baby at the end of pregnancy. And the events of childbirth, once begun, must progress rapidly to a conclusion or the life of a person giving birth and the baby are at risk. The extreme muscular work of labor and delivery are the result of a positive feedback system ( Figure 1.11 ).

The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors). These nerve cells send messages to the brain, which in turn causes the pituitary gland at the base of the brain to release the hormone oxytocin into the bloodstream. Oxytocin causes stronger contractions of the smooth muscles in of the uterus (the effectors), pushing the baby further down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby is born. At this point, the stretching of the cervix halts, stopping the release of oxytocin.

A second example of positive feedback centers on reversing extreme damage to the body. Following a penetrating wound, the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced perfusion (penetration of blood) to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut down and the person will die. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. As each step of clotting occurs, it stimulates the release of more clotting substances. This accelerates the processes of clotting and sealing off the damaged area. Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. This is an adaptive, life-saving cascade of events.

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33.11: Homeostasis - Homeostatic Process

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Learning Objectives

• Give an example and describe a homeostatic process.

Homeostatic Process

The human organism consists of trillions of cells working together for the maintenance of the entire organism. While cells may perform very different functions, the cells are quite similar in their metabolic requirements. Maintaining a constant internal environment with everything that the cells need to survive (oxygen, glucose, mineral ions, waste removal, etc.) is necessary for the well-being of individual cells and the well-being of the entire body. The varied processes by which the body regulates its internal environment are collectively referred to as homeostasis.

Homeostasis

Homeostasis, in a general sense, refers to stability, balance, or equilibrium. Physiologically, it is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change. Adjustment of physiological systems within the body is called homeostatic regulation, which involves three parts or mechanisms: (1) the receptor, (2) the control center, and (3) the effector.

The receptor receives information that something in the environment is changing. The control center or integration center receives and processes information from the receptor. The effector responds to the commands of the control center by either opposing or enhancing the stimulus. This ongoing process continually works to restore and maintain homeostasis. For example, during body temperature regulation, temperature receptors in the skin communicate information to the brain (the control center) which signals the effectors: blood vessels and sweat glands in the skin. As the internal and external environment of the body are constantly changing, adjustments must be made continuously to stay at or near a specific value: the set point.

Purpose of Homeostasis

The ultimate goal of homeostasis is the maintenance of equilibrium around the set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to revert to it. A change in the internal or external environment (a stimulus) is detected by a receptor; the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by moving the nutrient into tissues in the command center that require it, or to store it for later use.

• Homeostasis is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change.
• Homeostatic regulation is monitored and adjusted by the receptor, the command center, and the effector.
• The receptor receives information based on the internal environment; the command center, receives and processes the information; and the effector responds to the command center, opposing or enhancing the stimulus.
• homeostasis : the ability of a system or living organism to adjust its internal environment to maintain a stable equilibrium
• effector : any muscle, organ etc. that can respond to a stimulus from a nerve

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Homeostasis: The Underappreciated and Far Too Often Ignored Central Organizing Principle of Physiology

The grand challenge to physiology, as was first described in an essay published in the inaugural issue of Frontiers in Physiology in 2010, remains to integrate function from molecules to intact organisms. In order to make sense of the vast volume of information derived from, and increasingly dependent upon, reductionist approaches, a greater emphasis must be placed on the traditional integrated and more holistic approaches developed by the scientists who gave birth to physiology as an intellectual discipline. Our understanding of physiological regulation has evolved over time from the Greek idea of body humors, through Claude Bernard’s “milieu intérieur,” to Walter Cannon’s formulation of the concept of “homeostasis” and the application of control theory (feedback and feedforward regulation) to explain how a constant internal environment is achieved. Homeostasis has become the central unifying concept of physiology and is defined as a self-regulating process by which an organism can maintain internal stability while adjusting to changing external conditions. Homeostasis is not static and unvarying; it is a dynamic process that can change internal conditions as required to survive external challenges. It is also important to note that homeostatic regulation is not merely the product of a single negative feedback cycle but reflects the complex interaction of multiple feedback systems that can be modified by higher control centers. This hierarchical control and feedback redundancy results in a finer level of control and a greater flexibility that enables the organism to adapt to changing environmental conditions. The health and vitality of the organism can be said to be the end result of homeostatic regulation. An understanding of normal physiology is not possible without an appreciation of this concept. Conversely, it follows that disruption of homeostatic mechanisms is what leads to disease, and effective therapy must be directed toward re-establishing these homeostatic conditions. Therefore, it is the purpose of this essay to describe the evolution of our understanding of homeostasis and the role of physiological regulation and dysregulation in health and disease.

Introduction

In November 2009, I agreed to launch a new open-access physiology journal to be called Frontiers in Physiology and the articles were published in April 2010. One of my duties as Field Chief Editor was to write a brief “Grand Challenge” article in which I discussed what I perceived to be the biggest challenges facing physiology as a discipline. As it has been 10 years since the publication of this first essay, it is an opportune time to re-visit and update this grand challenge article.

The Grand Challenge in Physiology

In my 2010 essay, I stated that the grand challenge of physiology was “to integrate function from molecules to man” ( Billman, 2010 ). In other words, to make sense of the vast volume of information derived from, and increasingly dependent upon, reductionist approaches. This, in my opinion, remains the most serious unmet challenge facing physiology today. A greater emphasis must be placed on the traditional integrated and more holistic approaches developed by the scientists who gave birth to physiology as an intellectual discipline. In other words, it time for physiologists to return our roots. It is no more possible to appreciate the beauty of de Vinci’s “Mona Lisa” or Van Gogh’s “The Starry Night” by removing and analyzing each individual dab of paint than we can understand how the various organ systems work together to maintain health by examining single genes or molecules. Just as when viewing a painting, the body can only be fully appreciated in its entirety. This essay will focus on the concept of homeostasis as the central organizing principle upon which the discipline of physiology is built, the very concept we need to return to in order to integrate function from molecule to the intact organism. Portions of the following sections were previously published in a slightly different form ( Billman, 2013 ) and are reprinted with permission of the publisher.

Homeostasis: a Definition

Homeostasis, as currently defined, is a self-regulating process by which biological systems maintain stability while adjusting to changing external conditions. This concept explains how an organism can maintain more or less constant internal conditions that allow it to adapt and to survive in the face of a changing and often hostile external environment. Our awareness of homeostasis has slowly emerged over the centuries and has become the central organizing tenet of physiology. If one does not understand this self-regulating process, then it is not possible to comprehend fully the function of the body in health and in disease. The disruption of homeostatic mechanisms is what leads to disease, and effective therapy must be directed toward re-establishing these homeostatic conditions, working with rather than against nature. In the following sections, the evolution of our understanding of homeostasis will be described and the role of physiological regulation and dysregulation in health and disease will be evaluated.

Homeostasis: a Historical Perspective

“ True stability results when presumed order and presumed disorder are in balance. A truly stable system expects the unexpected, is prepared to be disrupted, waits to be transformed. ” Tom Robbins (American Novelist, b. 1936) 1

The concept that bodily regulation is required for health can be traced back to the ancient Greeks. The Greek physician/philosopher Alcmaeon of Croton (fl. 500 BC) proposed what can be called a “balance of opposites” to explain health and disease. He used a political analogy to define health and disease stating that: “ Health is the equality of rights of the functions, wet-dry, cold-hot, bitter-sweet and the rest; but single rule of either pair is deleterious. ” ( Freeman, 1948 ). Thus, inequality of power leads to tyranny in a political system and disease in the body. This concept was expanded by Hippocrates of Kos (ca. 460–ca. 377 BC) who proposed that health was the product of the balance and mixture of four body fluids or humors: blood, phlegm, yellow bile, and black bile. He wrote that:

“ Health is primarily that state in which these constituent substances are in correct proportion to each other, both in strength and quantity and are well mixed. Pain occurs when one of these substances presents either a deficiency or excess, or is separated in the body and not mixed with the others. ” ( Chadwick and Mann, 1950 )

Thus, medicine became a process “ of subtraction and addition: subtraction of what is in excess, addition of what is wanting. ” ( Jones, 1923 ). Hippocrates further recognized the role of nature’s helping hand in the healing process ( vis medicatrix naturae ), the ability of the body to heal itself ( Hall, 1975 ). It was the role of the physician to clear the path so that nature could take its course. This concept became the basis for medicine in the ensuing centuries up to the dawn of the modern era.

Implicit in this concept of the “healing power of nature” is the assumption that the subunits of the body act in a cooperative manner to restore health when the normal state of the organism has been disturbed. Physiology, as a discipline dedicated to understanding how the parts of the body work together to maintain health, has its origins in the 16th century. The term physiology was first introduced by Jean Francois Fernel (ca. 1497–1558, Figure 1 ) in 1542 [ De Naturali Parte Medicinae (on the natural part of medicine)] as the study of the function of the healthy body as distinguished from pathology, the study of disease ( Hall, 1975 ). William Harvey (1578–1657) was the first individual to use carefully designed human and animal experiments to establish the function of a major bodily organ system with his description of the circulation of the blood. This application of physiology is illustrated in the following brief quotation from his seminal publication “ Exercitatio Anatomica De Motu Cordis et De Circulatione Sanguinis in Animalibus ” 1628 (Anatomical exercises on the motion of the heart and the circulation of blood in living creatures, first English translation 1653):

Portrait of Jean Fernel (ca. 1497–1558). He is the individual who coined the term physiology. Source: National Library of Medicine (the history of medicine public domain image files).

“It has been shown by reason and experiment that blood by the beat of the ventricles flows through the lungs and is pumped to the whole body … the blood in the animal body moves around in a circle continuously, and … the action or function of the heart is to accomplish this pumping. This is the only reason for the motion and beat of the heart.” ( Harvey, 1628/1653 )

Over the ensuing centuries, the concept of physiology has evolved, and a central tenet has emerged that unites the various sub-disciplines of physiology: the quest to understand how the various components of the organism work together to maintain a healthy state. It is only by understanding normal bodily function that the disruptions that lead to disease can be determined and ultimately corrected so as to restore the healthy state.

As we have seen, a rudimentary understanding of the regulation and control of bodily function can be traced back to 6th century BC Greece. Despite sporadic progress over the centuries ( Adolph, 1961 ), it was not until the 19th century that systematic physiological investigation produced major advancements on this concept. Our modern understanding of physiological regulation rests firmly on the shoulders of two giants in the field: Claude Bernard ( Figure 2 ) and Walter Cannon ( Figure 3 ) who described regulations in terms of the constancy of the internal environment and homeostasis, respectively.

Photograph of Claude Bernard (1813–1878). He developed the concept of “ a fixité du milieu intérieur ,” that is, organisms maintain a stable internal environment despite changing external conditions. Source: National Library of Medicine (the history of medicine public domain image files).

Photograph of Walter B. Cannon (1871–1945). He built upon the work of Claude Bernard and coined the word homeostasis to describe a self-regulating process by which biological systems maintain stability while adjusting to changing conditions. Source: National Library of Medicine (the history of medicine public domain image files).

The French Physiologist, Claude Bernard (1813–1878), who is often referred to as the founder of modern experimental physiology, was perhaps the first to appreciate fully that living systems possess an internal stability that buffers and protects the organism against a constantly changing external environment ( Cooper, 2008 ). He recognized that the body possesses mechanisms that operate in a coordinated fashion to maintain a relatively constant temperature and blood glucose concentration and this internal stability was vital for the health of the organism. He concluded that: “ La fixité du milieu intérieur est la condition de la vie libre, independante ” ( Bernard, 1865 ) [The fixity (i.e., constancy or stability) of the internal environment is the condition for the free, independent life]. What is often overlooked and needs to be stressed is that in this statement Bernard was proposing a new and radical hypothesis: the stability of the “ milieu intérieur ” was the antecedent to (i.e., required for) and not the consequence (outcome) of a free and independent life ( Turner, 2017 ).

Although Bernard was highly honored and was the most famous French scientist during his lifetime, his hypothesis that the stability of the internal environment was independent of the external conditions, first articulated in 1854, was largely ignored for the next 50 years. Gross (2009) has proposed three reasons to explain the delay between the publication of Bernard’s ideas and their acceptance: (1) Pasteur’s exciting discoveries in bacteriology that had immediate application in the prevention and treatment of disease came to dominate biological investigations; (2) the gap between evolutionary thought and general physiology—it took time to appreciate that natural selection provided the means by which regulatory control could evolve; and (3) the technology necessary to measure the internal environment was not yet available.

However, by the late 19th century and early 20th century several investigators embraced Bernard’s ideas, both as a central explanatory concept and as a program for research in physiology. Among those influenced by Bernard were such physiological luminaries as William M. Bayliss, Ernest H. Starling, Joseph Barcroft, J. S. Haldane, and C. S. Sherrington in England, and L. J. Henderson and Walter B. Cannon in America ( Adolph, 1961 ; Cooper, 2008 ; Gross, 2009 ). Starling, in fact, coined the phrase “the wisdom of the body” to describe the maintenance of a constant internal environment ( Cooper, 2008 ). Walter Cannon later popularized this phrase when he used it as the title for his book in which he introduced the concept of homeostasis. In 1900, Charles R. Richet (1850–1935), a student of Bernard who later won the Nobel Prize in Physiology and Medicine, stressed the dynamic stability of the internal environment. The following quote, we shall see, presaged the definition supplied by Walter Cannon.

“ The living system is stable … it must be in order not to be destroyed, dissolved or disintegrated by colossal forces, often adverse, which surround it. By an apparent contradiction, it maintains its stability only if it is excitable and capable of modifying itself according to external stimuli and adjusting its response to the stimulation. In a sense, it is stable because it is modifiable – the slight instability is the necessary condition for the true stability of the organism. ” ( Richet, 1900 )

This concept of a constant internal environment ( milieu intérieur ) was expanded by the American Physiologist, Walter Cannon (1871–1945) ( Cooper, 2008 ). He coined the term homeostasis from the Greek words Ǒμoιoς (hómoios) “similar” and στάσις (stásis) “standing still” (together to mean staying similar and not staying the same) to describe the self-regulating processes by which a biological system maintains stability while adjusting to changing environmental conditions. Homeostasis is often mistakenly taken to mean unchanging or stagnant. However, Cannon purposely selected the Greek word for similar, “hómoios,” rather than the word for same, “homo,” to express the idea that internal conditions could vary; that is, they are similar but not identical (stability but within range of values that allows the organism the freedom to adapt). Homeostasis, then, is the tendency of a system to maintain an internal stability as the result of the coordinated response of its parts to any situation or stimulus that disturbs normal conditions or function. Thus, the term homeostasis attempts to convey two ideas: (1) an internal stability within a range of values and (2) the coordinated dynamic response that maintains this internal stability (self-regulatory goal-seeking behavior). As he explained in the following quote from his highly influential monograph, “The Wisdom of the Body,” published in 1932:

“ The coordinated physiological processes which maintain most of the steady states in the organisms are so complex and peculiar to living beings – involving, as they may, the brain and nerves, the heart, lung, kidneys and spleen, all working cooperatively – that I have suggested a special designation for these states, homeostasis. The word does not imply, something set and immobile, a stagnation. It means a condition – a condition which may vary, but is relatively constant. ” ( Cannon, 1963 )

As emphasized by Cannon, homeostasis is not static; it is, rather, a dynamic self-adjusting system that maintains viability in the face of changing environmental demands. Echoing Bernard, homeostasis is a unique property of living organisms and, may be responsible for life itself. More recently, Turner (2017) described homeostasis as a dynamic disequilibrium – dynamic, as a stable internal environment requires continuous monitoring and adjustments (once again, a self-regulatory process) in order to maintain a balance between opposing forces (what he calls disequilibrium) so that a free and independent life is possible. He went further and stated that “ properly understood, homeostasis is life’s fundamental property, what distinguishes it from non-life. In short, homeostasis is life. ” ( Turner, 2017 ).

The final piece of the homeostasis puzzle was supplied by the application of control theory from systems engineering to explain self-regulation in biological systems. The “constancy” of internal physiochemical conditions is then largely maintained by the often complex interaction of multiple negative (and positive) feedback systems. The interaction of these regulatory mechanisms not only increases the stability of the system but provides redundancy (back-up) such that failure of one component does not necessarily lead to catastrophe. Thus, from its inception physiological investigations have been directed toward understanding the organism (be it microbe, plant, animal, or human) as a single functional entity .

Feedback Regulation: the Process That Underlies Homeostasis

“ Nam deteriores omnes sumus licentiate. ” ( We all degenerate in the absence of control ) Terence (Heauton Timorumenos, line 483)

As we have seen, a critical feature of homeostasis is that an organism’s internal environment is held within a narrow range of values via a self-adjusting (a goal-seeking) system. Both feedback and feedforward are the mechanisms by which homeostasis is obtained. I shall begin this section with a discussion of the contribution of feedback to homeostatic regulation and then briefly discuss feedforward (also known as central command) mechanisms.

A feedback system is a closed loop structure in which the results of past actions (changes in the internal environment) of the system are fed into the system (via information, feedback) to control future action; the system affects its own behavior (modified from Forrester, 1976 ). There are two types of feedback systems: negative feedback that seeks a goal and responds as a consequence of failure to meet this goal (maintains a stable range of values) and positive feedback that produces growth processes wherein the actions build on the results that then generate still greater action (a growth cycle). These feedback systems are themselves subject to higher levels of control; that is, the operational range of the regulated variables can be adjusted to support the behavioral response to environmental stimuli. Homeostasis is the result of the complex interaction and competition between multiple negative and positive feedback systems and provides the basis for physiological regulation.

Once again we can trace the origin of self-regulatory systems to the ancient Greeks.

The first documented device that employed the principle of self-regulation was a water clock (clepsydra) invented by Ktesibios (or Ctesibius, Greek Kτησίβις) of Alexandria (fl. 285-222 BC) ( Landels, 2000 ). A water clock depends upon a steady flow of water to measure an unvarying flow of time. If the water level is not relatively constant, the water outflow will vary depending on the height of the water column supplying the clock (faster with a full container and slower as the water level in the container falls). The water clock designed by Ktesibios used a float valve (similar to that used in the modern flush toilet) to maintain a constant water level in the clock water reservoir. As water levels fall, the float also falls thereby opening a valve that allows water to flow into the clock reservoir and to replenish the water level. Then, as the water returns to the desired level, the float rises and closes the valve. Thus, the clock water reservoir could be regulated such that there is no net gain or loss in the water level and thereby it maintains a constant water outflow rate from which an accurate estimate of time can be obtained. The accuracy of this type of water clock was not supplanted until the 17th century when a pendulum was employed to regulate the clock mechanism.

A number of other self-regulatory devices were invented in the ancient and medieval periods but it was not until the late 18th century, with the invention of the steam engine that the study of devices that incorporated “corrective feedback” for regulation became a subject for systematic investigation. A major limitation of early steam engines was that their speed was affected by both the steam pressure generated by the boiler and work load placed upon the engine. James Watt (1736–1819) vastly improved the efficiency and safety of the steam engine by the development of a centrifugal feedback valve that controlled the speed of the engine ( Rosen, 2010 ). This “governor” ( Figure 4 ) employed a pair of metal balls spinning on each side of a rotating vertical shaft aligned in such a manner that as the engine speed increased so also did the spinning rate of metal balls (called flyweights) and, as a consequence of increased centrifugal force, the balls would spread apart. This, in turn, opened a valve to decrease the flow of steam into the engine and a slower speed was restored. Conversely, as the engine speed decreased, so also would the rotation of the flyweights, thereby decreasing the outward centrifugal force. The flyweights would drop (pulled down by gravity) closer together, closing the steam valve so more steam could enter into the engine and increase its speed. As with the water clock and its water reservoir level, a constant engine speed could be maintained despite fluctuating steam pressure and changing work load without the constant supervision of a human monitor.

Schematic representation of James Watt’s steam engineer flyweight governor. See text for details. Source: public domain, as modified from, https:www.mpoweruk.com/figs/watt_flyball_governor.htm .

Later in the 19th century, James Clerk Maxwell (1831–1879) published a mathematical analysis of Watt’s governor that established the principles for understanding self-regulating devices and became the foundation upon which control theory is built ( Maxwell, 1868 ). In 1927, Harold S. Black (1898–1983) applied feedback regulation to electrical circuits to amplify transatlantic telephone signals ( Black, 1934 ). His negative feedback amplifier (patented in 1937) can be considered to be one of the most important developments in the field of electronics. Further advances in systems control theory were achieved during World War II with the development of servo-control (negative feedback) mechanisms for anti-aircraft weapons.

In 1943, two influential papers were published that established that the mathematical principles of control theory, as first described by Maxwell, could be applied to explain behavior in living organisms. Arturo Rosenblueth, Norbert Wiener, and Julian Bigelow’s paper entitled “Behavior, Purpose and Teleology” ( Rosenblueth et al., 1943 ) and Warren McCulloch and Walter Pitts’, “A Logical Calculus of the Ideas Immanent in the Nervous Activity” ( McCulloch and Pitts, 1943 ) were the first to establish a link between the self-regulating nature of physiological processes in living animals and negative-feedback systems designed by engineers. Interestingly, Rosenblueth worked closely with Cannon and undoubtedly was influenced by his ideas. A few years later, Wiener (1894–1964) introduced the term cybernetics [from kybernetes (κυβερνήτης), the Greek word for governor (as in steersman or pilot)] to describe the study of self-regulatory control and communication in the animals ( Wiener, 1961 ). In his book Cybernetics, Wiener (1961) developed the first formal mathematical analysis of feedback control in biological systems, concepts that have subsequently been extensively applied in modeling physiological systems as, for example, by Arthur Guyton (1919–2003) and his many students with regard to cardiovascular regulation. Thus, the concept of feedback regulation in living organisms may be said to have co-evolved with the mathematical concepts of control theory in mechanical systems. Negative feedback regulation is a particularly important mechanism by which homeostasis is achieved, as will be described in the following paragraphs.

The water clock and centrifugal steam governor described in the preceding paragraphs provide classic examples of negative feedback systems. As we have seen for the water clock, the opening and closing of the float/valve creates a cycle where information about the water level can be fed back into the system to effect changes to maintain the water level at some constant pre-determined value. Thus, the float simultaneously affects the water levels and is affected by water level forming a circular causality or a cycle of causation. It is important to emphasize that this is an automatic self-regulatory system, meaning that it requires no external adjustment once the operating level around which the variable is regulated has been set.

A simplified general form of a closed loop feedback system is illustrated in Figure 5 . The illustrated cycle consist of four main components, (1) the variable (or set of variables) that are to be controlled, (2) a sensor that monitors the variable of interest, (3) a comparator or central processing unit (mathematically, the transfer function—the input/output relationship) where the information provided by the sensor (afferent or sensory pathway) is fed back into the system. The information is compared with the “desired” state (set point or operating point) to detect any error (difference between the desired state and the prevailing state), and (4) effectors (efferent or motor pathways) that are activated to correct any error. Effector activity opposes and thereby buffers against changes in the variable. A solid line is used in this diagram to indicate a direct relationship (increase leads to increase, decrease leads to decrease) between the components, while a dashed line represents an inverse relationship (increase leads to a decrease and vice versa). Negative feedback regulation must contain an odd number of dashed lines in order to maintain the variable within a narrow range of the desired value.

A schematic representation of negative feedback regulation. A solid line indicates that the connected components are directly related (an increase in one component leads to increase the connected component, while a decrease will lead to decrease in the connected components). A dashed line indicates the connected components are inversely related (an increase in one component leads to a decrease in the connected component while a decrease will lead to an increase in the connected component). An odd number of dashed lines are a necessary condition for any negative feedback cycle of causation. Negative feedback acts to maintain the controlled variable within a narrow range of values (see text for a detailed description).

A commonly used example of negative feedback is the regulation of room temperature by a thermostatically controlled heating and cooling system as displayed in Figure 6 . Room temperature is the regulated variable, the sensor is a thermometer, the comparator is the thermostat—the device that compares the desired temperature (operating point) with the actual temperature (error detection), and the effector is the heating or cooling system. In this example, an increase in outside heat is detected by the sensor and the information is conveyed to the thermostat. The temperature information is compared to operating point and if there is sufficient difference between actual and desired temperature, the cooling system is activated and the heating system is inactivated (reducing the error signal). The converse would happen if environmental temperature should fall, the cooling system would be turned off and the heating units activated. Thus, stable room temperatures can be maintained despite a wide range of fluctuating external conditions.

A schematic representation of the regulation of room temperature to illustrate the concept of negative feedback regulation. A solid line indicates that the connected components are directly related (an increase in one component leads to an increase the connected components, while a decrease will lead to a decrease in the connected components). A dashed line indicates that the connected components are inversely related (an increase in one component leads to a decrease in the connected component while a decrease will lead to an increase in the connected component). Negative feedback acts to maintain the room temperature within a narrow range of values despite changes in ambient temperature (see text for a detailed description).

It must be emphasized that feedback regulation in biological systems (living organisms) is much more complex than the simple “clockwork” feedback systems described in the preceding paragraphs for mechanical systems. With this caveat firmly in mind, the concept of self-regulation in biological system can be illustrated by the regulation of blood pressure. As early as the mid-19th century, it became obvious that arterial blood pressure was maintained within a narrow range of values via the activation of neutrally mediated reflex adjustments ( Adolph, 1961 ). However, it was not until to 1960s that the principles of negative feedback were applied to explain the homeostatic regulation of arterial blood pressure. A detailed description of intricacies of blood pressure regulation is beyond the scope of the present essay (for a recent review see Dampney, 2016 ). Nonetheless, a simplified feedback cycle, analogous to the one we used for room temperature, is seen in Figure 7 .

A simplified schematic representation of the regulation of arterial blood pressure as a physiological example of negative feedback regulation. A solid line indicates that the connected components are directly related (an increase in one component leads to an increase the connected components, while a decrease will lead to a decrease in the connected components). A dashed line indicates the connected components are inversely related (an increase in one component leads to a decrease in the connected component while a decrease will lead to an increase in the connected component). Negative feedback regulation acts to maintain the arterial blood pressure within a narrow range of values (see text for a detailed description). NTS = nucleus tractus solitarius, the site where sensory information is processed and the efferent response is initiated. It acts as a “barostat” analogous to the “thermostat” in room temperature regulation. SV = stroke volume (the amount of blood ejected by the heart with each ventricular contraction), HR = heart rate, the number of beats (ventricular contractions) per minute, TPR = total peripheral resistance, the resistance to the forward movement of blood (inversely related to the blood vessel diameter).

Before we can discuss this figure, we first must mathematically define arterial pressure using Ohm’s law expression (for a hydraulic rather than for an electrical circuit). Algebraically, blood pressure (BP – analogous to voltage, E, in an electrical circuit) is the product of the cardiac output (CO – analogous to current, I, in an electrical circuit) and systemic vascular resistance also known as total peripheral resistance (TPR – analogous to electrical resistance, R). Cardiac output is itself the product of the amount of blood ejected per beat [stroke volume (SV)] multiplied by the number of beats per minute [heart rate (HR)].

So that, BP = SV × HR × TPR. (E = I × R for an electrical circuit).

It is evident that changes in arterial blood pressure can be countered by corrective changes in either the output from the heart (SV and/or HR) or resistance to movement of blood through blood vessel (by adjusting vessel diameter, diameter is inversely related to TPR) or both. Returning to Figure 7 , the sensors are receptors (baroreceptors) located in arterial blood vessels (aortic arch and carotid sinuses) that respond to changes in arterial pressure (increases in BP increase receptor activity). The comparator function is performed by a cluster of nerve cells within the medulla of brain [nucleus tractus solitarius (NTS)] where the signal is processed to affect the output of the effector system. It acts as a “barostat” a function analogous to the thermostat in the regulation of room temperature shown in Figure 6 . The signal is processed at the NTS and then effects excitatory [rostral ventral lateral medulla (RVLM) via interneuron connections] and inhibitory [nucleus ambiguus (NA), monosynaptically] areas within the medulla to elicit the motor response (see Figure 8 for more details). The motor output from the central nervous system to target organs is conducted by means of two sets of nerves to the heart: parasympathetic nerves (originating in the NA) that decrease HR and sympathetic nerves (originating in the intermediolateral column, IML of the spinal cord, regulated by neurons from the RVLM) that increase HR and SV. The sympathetic nerves also go to blood vessels, the activation of which decreases vessel diameter and thereby increases TPR. Thus, if BP should increase, the so-called baroreceptor reflex is activated. An increase in parasympathetic activity coupled with a decrease in sympathetic activity would reduce cardiac output (decreasing HR and SV) and decrease TPR. The opposite changes would occur if blood pressure should decrease. Thus, negative feedback regulation buffers against transitory changes and thereby helps maintain a stable blood pressure on a beat-by-beat basis throughout the day despite changing environmental or behavioral conditions.

A simplified schematic representation of the central neural structures involved in baroreceptor reflex regulation of arterial blood pressure. Arterial pressure receptors located in the carotid sinuses and aortic arch (nerve firing increases as arterial pressure increases) convey afferent information via the glossopharyngeal (IXth) and vagus (Xth) nerves to the brain, respectively. This information is first processed by neurons located in the nucleus tractus solitarius (NTS). The NTS then alters parasympathetic and sympathetic efferent nerve activity. Specifically, the NTS alters the activity of neurons (monosynaptically) located in the nucleus ambiguus (NA, parasympathetic pre-ganglionic neurons) and neurons (polysynaptically, via interneuron connections) in the caudal ventrolateral medulla (CVLM). The CVLM, in turn, regulates the tonic sympathetic activity that originates in the rostral ventrolateral medulla [RVLM, that regulates sympathetic pre-ganglionic neurons located in the intermediolateral column (IML) of the spinal cord]. + = excitatory neurotransmitters (shown in black); – = inhibitory neurotransmitters (shown in blue); SAN = sino-atrial node. As an example, an increase in arterial blood pressure would increase baroreceptor nerve firing, increasing NTS neuron activity which, via interneurons, would trigger both an increase in the activity of the parasympathetic pre-ganglionic neurons located in the NA and decrease the firing of sympathetic pre-ganglionic neurons located in the IML (less directly via CVLM mediated inhibition of the tonic activity of the RVLM). The net result would be a decrease in heart rate (? cardiac parasympathetic and↓ cardiac sympathetic nerve activity), stroke volume (↓ cardiac sympathetic nerve activity), and arteriolar vasoconstriction (↓ total peripheral resistance, ↓ cardiac sympathetic nerve activity). Reductions in arterial blood pressure would provoke changes in the opposite direction. Note that the sign changes at the heart (parasympathetic effects on the SAN) and within the medulla (CVLM mediated inhibition of the RVLM). This “sign change” is necessary for negative feedback regulation.

Feedforward regulation is another mechanism by which homeostasis is modified and maintained as part of the behavioral response to environmental stimuli. During feedforward regulation, which is also often referred to as central command, a response is elicited without feedback about the status of the regulated variable; that is, disturbances are evaluated and adjustments are made before changes in the regulated variable have actually occurred. For example, returning to constant room temperature, feedforward regulation would entail activation of the furnace as soon a window or door is opened during a cold winter day before the thermostat detects a change in the ambient temperature, In a similar manner, blood pressure, cardiac output, and skeletal muscle blood increase in anticipation of fighting or fleeing a potential danger (the defense reaction) or when an athlete envisions running the race before the starter’s pistol has been fired (see below). It should be emphasized that feedforward regulation, while acting independently of changes in the regulated variable, does require information about the nature and extent of the potential disturbance. For room temperature, the status of the windows and doors (whether they are open or not) must be monitored (sensors placed on these openings). Otherwise, a response would not be elicited until room temperature had deviated sufficiently from the set point to be detected by the thermostat (and thereby activate the previously described negative feedback response). In living organisms, learning and experience provide the information necessary for feedforward control. A cat soon learns the difference between a mouse (food) and the neighbor’s dog (a dangerous and barking nuisance) and will react accordingly (making the appropriate behavioral and physiological adjustments for appetitive or aversive stimuli).

The simple negative feedback schema described in the preceding paragraph cannot adequately convey the complexity of the homeostatic process that allows an organism to function and adapt to changing environmental conditions ( Carpenter, 2004 ). For example, the operating point (or more accurately the operating range) of the negative feedback regulation can be adjusted or even overridden by higher levels of control ( Goodman, 1980 ). These adjustments of the automatic (e.g., feedback) regulation allow the organism to adapt and to respond appropriately to changing external conditions. This hierarchical control is a multi-level, multi-goal seeking system as shown in Figure 9 (modified from Goodman, 1980 ). In this schematic diagram, the first level represents the physiochemical processes, the organ and tissue functions, the component parts upon which homeostasis acts. The second level is autonomous (self) regulation, homeostasis (e.g., baroreceptor reflex). Here changes in a given variable are sensed and adjustments of the first level processes are initiated without input from higher levels of control. The third level is found in the central command and control centers (central nervous system) that process the information transmitted from the second level and integrates it with information from other sensory inputs to coordinate the physiological and behavioral response to changing environmental conditions. The higher centers can “intervene,” making the adjustments as required to support the autonomic (i.e., autonomous and automatic) processes. This control can occur either at the conscious or unconscious level. An example of a conscious intervention would be the initiation of behaviors to cope with changing room temperature – adding or removing clothing, opening or closing windows seeking shade or sun, etc. – while an example of subconscious control would be the adjustments in blood pressure regulation during exercise (a shift in the operating point of the baroreceptor reflex so that both HR and SV increase despite increases in BP as compared to resting conditions; Raven et al., 2006 ). Thus, the third level coordinates behavioral and physiological responses to the external environment in order to maintain comfort and to ensure survival. However, it must be emphasized that higher level control is not possible if the first level components do not function properly. Finally, one could also envision even higher levels of control, factors outside of the organism.

A simplified schematic representation of the higher order control of homeostatic regulation. This hierarchical control results in a finer level of control and a greater flexibility that enables the organism to adapt to changing environmental conditions (see text for details). CNS = central nervous system.

The “autopilot” in a modern jet airliner can be used to illustrate the levels of control ( Wiener, 1961 ). Once the preferred heading, attitude, and airspeed have been set, the autopilot will maintain level flight within acceptable degrees of roll, pitch, and yaw, despite changes in wind speed or minor turbulence. However, take-off and landing (at least until “self-driving” technology has been perfected) require the direct intervention of the human pilot. Thus, the first level consists of the components of the airliner, the jet engines, and the airframe (fuselage, wings, flaps, rudder, etc.), the second level is the autopilot, and third level is the human pilot. In this example, a fourth level of control of the airplane is exerted by the air traffic controllers who provide directions to the pilot while an even higher level of control would reside in the Federal Aviation Administration (FAA) that sets the policy followed by the air traffic controllers.

The cardiorespiratory response to exercise provides a physiological example of this hierarchical control of homeostatic regulation. The first level consists of the tissues and organs that form the cardiovascular and respiratory system (heart, lung, and blood vessels, but also the kidneys and endocrine glands that regulate salt and water retention and thereby blood volume), the second level of control is the baroreceptor (direct effect) and cardiorenal reflexes (indirect via regulation of blood volume), the third level of regulation takes place within the medulla (NTS) of the central nervous system where the sensory information is processed and the efferent response initiated. The medullary structures are themselves regulated by higher centers (e.g., hypothalamus and motor centers) in the brain. In fact, the hypothalamus plays a major role in coordinating (matching) changes in the internal environment with the behavioral response to external challenges. As previously mentioned, HR and BP are simultaneously elevated during exercise demonstrating that baroreceptor reflex regulation has been altered. These adjustments are required in order to increase oxygen delivery so that it can match the increased metabolic demand of the exercising muscles. Raven et al. (2006) have demonstrated that these adjustments result from shifting the baroreceptor reflex to a higher operating point (i.e., altering the range of homeostatic regulation) rather than from an inhibition of this reflex. Both feedback (sensory information for the exercise muscle, the so-called exercise pressor reflex) and feedforward (central command: for example, anticipation of the onset of exercise, such as visualizing the race before it is run, will increase HR, BP, and skeletal muscle blood flow) contribute to these reflex adjustments. Finally, higher levels of control include the starter who determines when the race will begin, the event organizers who determine what races are run, and the sports regulatory agencies (Olympic committee, FIFA, NCAA, etc.) that set the rules that govern the event.

Homeostatic control of the internal environment, therefore, involves much more than simple negative feedback regulation ( Carpenter, 2004 ). The hierarchical levels of command and control allow the organism to adjust its internal conditions to respond, to adapt, and to meet the challenges placed upon it by a changing and often hostile environment. Adaptation can, in fact, be viewed as an emergent property of homeostasis and may be responsible for the life’s unique nature ( Turner, 2017 ).

Homeostasis: Implication for Reductionism

“… All the kings’ horses and all the kings’ men Could not put Humpty Dumpty together again ” Traditional English Nursery Rhythm (earliest published version 1803) ( Opie and Opie, 1997 )

The concept of homeostasis has important implications with regard to how best to understand physiology in intact organisms. In recent years, reductionist (attempts to explain the nature of complex phenomena by reducing them to a set of ever smaller and simpler components; the view that the whole is merely the sum of its parts), rather than holistic approaches have become dominant, not only in physiology, but in science in general. The earliest glimmerings of reductionist thought can be found in the surviving fragmentary writings of Thales and other pre-Socratic Greek philosophers who speculated that all matter was composed of various combinations of four key elements: earth, air, fire, and water (the four humors of the body correspond to these elements) ( Hall, 1975 ). The pinnacle of Greek reductionism is found in the work of Leucippus and his student Democritus who proposed that all things consist of an infinitely large number of indivisibly small particles that they called atoms ( Hall, 1975 ). The modern application of reductionism in science can be traced to Francis Bacon (1561–1620) and Rene Descartes (1596–1650). Bacon incorporated reductionism as a central component, along with inductive reasoning, in his new empirical method ( Novum Organum 1620, as opposed to Aristotle’s Organon a treatise on logic and syllogism, i.e., deductive reasoning) ( Bacon, 1620 ) for the attainment of knowledge in natural philosophy, what has subsequently become known as the scientific method. Descartes likewise embraced reductionism as the pathway to knowledge, albeit with an emphasis on deduction (rationalism) rather than induction (empiricism) as advocated by Bacon. In his “Discourse on the Method of Rightly Conducting One’s Reason and Seeking Truth in Science,” Descartes (1637) introduced two concepts that would have profound impact on biological investigations. In this, his most influential treatise, he described four precepts to arrive at knowledge. The second and third precepts, in particular, exemplify the reductionist’s approach as follows:

“ The second to divide each of the difficulties under examination into as many parts as possible and as might be necessary for its adequate solution ” “ The third to conduct my thoughts in such order that, beginning with those objects that are simplest and most readily understood, I ascend little by little, and as it were, step by step, to the knowledge of the more complex. ” ( Descartes, 1637 )

His second and more far reaching conclusion was that the body was merely a machine. Thus, it was assumed that by applying Cartesian reductionism, one could deduce the complex physiology of the intact organism by understanding the presumably simpler functions of the individual organs and their constituent parts (from the molecular level to subcellular organelles to cells to tissue to organ and finally back to the intact organism).

There can be no denying the power of this approach. In only a few decades after DNA was identified as the molecule of inheritance, its sequence of the some 3 billion base pairs has been mapped for humans and other species, the genetic “code” for protein synthesis has been broken, and between 20,000 and 25,000 human genes that regulate a multitude of proteins have been determined. Humpty Dumpty quite literally has been smashed into a billion pieces.

However, reductionism rests upon the unstated assumption that the parts somehow entail the whole, that complexity is merely the product of incomplete understanding. In other words, the assumption that once we have gathered enough information (big data) and have developed sufficient computing power (ultra-fast computers), we can put Humpty back together again. The salient question is then whether this assumption is correct? Although we have sequenced the genome for many species, we have little understanding of the process by which the genome becomes an organism. We now know, in intricate detail, the basis for neuronal action potentials and synaptic transmission but do not understand how these electrical and chemical events give rise to consciousness. Complexity may not be the illusion it once naïvely was thought to be. As elegantly described by Claude Bernard more than 150 years ago:

“ Physiologist and physicians must never forget that a living being is an organism with its own individuality. Since physicists and chemists cannot take their stand outside the universe, they study bodies and phenomena in themselves and separately, without necessarily having to connect them with nature as whole. But physiologists, finding themselves, on the contrary, outside the animal organism which they see as a whole, must take account of the harmony of the whole, even while trying to get inside, so as to understand the mechanism of its every part. The result is that physicists and chemists can reject all idea of the final causes for the facts that they observe; while physiologists are inclined to acknowledge a harmonious and pre-established unity in an organized body, all of whose partial actions are interdependent and mutually generative. We really must learn, then, that if we break up a living organism by isolating its different parts, it is only for the sake of ease in experimental analysis, and by no means in order to conceive them separately. Indeed, when we wish to ascribe to a physiological quality its value and true significance, we must always refer to this whole, and draw conclusions only to its effects in the whole. ” (Emphasis added, Bernard, 1865 )

It cannot be overstated that the whole is greater than the sum of the parts!

The grand challenge faced by contemporary physiology in this post-genomic era as first described in 2010 ( Billman, 2010 ) remains how to integrate and to translate this deluge of information obtained in vitro into a coherent understanding of function in vivo . Although a machine may consist of many parts, the parts in isolation do not make the machine. Anyone who has tried to assemble a child’s bicycle on Christmas Eve can testify that the parts do not a machine make. In an analogous fashion, while organisms are made of molecules, molecules are not organisms. The concept of one gene, one protein, one function is woefully inadequate to explain the dazzling complexity and startling beauty of the living organism – the intricate dance of homeostatic mechanisms necessary for a “free and independent life.” A sequence of base pairs in the DNA molecule can no more explain the complexities of life than a series of 1s and 0s on a compact disc recording can explain the emotional response to music ( Noble, 2006 ). Man and other organisms are not mere vehicles for the perpetuation of genes, selfish or otherwise. The days for reductionist deconstruction are numbered; more holistic and integrated systems approaches are required to put Humpty Dumpty back together again. It is time for physiologist to return to their roots and consider the organism as a whole as advocated by Claude Bernard.

A second, and by no means less important, challenge will be to train the next generation of scholars to perform the integrative studies in intact preparations (whole animals or organs) that are the pre-requisite for clinical applications. Unfortunately, there has been a progressive decline in the number of integrative physiology training programs, resulting in a paucity of individuals with the skill sets necessary for whole animal in vivo experimentation. The problem is exacerbated by the renaming or actual elimination of Departments of Physiology within Colleges of Medicine. It currently is fashionable for physiology departments to rechristen themselves as “Departments of Molecular Biology/Physiology.” With tongue firmly in cheek, one wonders if Departments of Atomic Physiology will be soon in the offing.

With the increasing emphasis on molecular and genetic approaches, it is not unusual to find members of physiology departments who have not even taken an introductory course in physiology. This is, indeed, a shame as much of the excitement for physiology as an intellectual discipline can best be encountered in the student lab. Nothing can replace the hands-on learning nor instill a better appreciation for the concept of homeostasis than performing these classic physiology experiments. In the student lab, one can go beyond the dry textbook description of physiological principles and see them in action. The student can experience, first hand, the same excitement and sense of wonder that the earlier investigators must have had when they first examined skeletal muscle-nerve function in frogs, saw the clearance of dye in the easily visible glomeruli in the necturus (mudpuppy), or pondered the mysteries of cardiopulmonary regulation in mammals (rat, rabbit or dogs). Thus, it very much remains an open question as to whether a sufficient number of suitably trained investigators will be available to meet the grand challenge: to integrate function from molecules to intact organisms.

Our understanding of physiological regulation has evolved over time from the Greek idea concerning the balance between the body humors, through Claude Bernard’s “milieu intérieur” to Walter Cannon’s formulation of the concept of homeostasis and the application of control theory (feedback regulation) to explain how a constant internal environment is achieved. Homeostasis has become the central unifying concept of physiology and is defined as a self-regulating process by which a living organism can maintain internal stability while adjusting to changing external conditions. Homeostasis is not static and unvarying; it is a dynamic process that can change internal conditions as required to survive external challenges. This is made clear by the care Cannon used when coining the word homeostasis. He deliberately selected Greek words that when, combined, meant “staying similar” rather than “staying the same” to emphasize that internal conditions could vary yet still produce stability (within a range of values rather than a single value). Thus, homeostasis does not mean “stagnation.” It is also important to note that homeostatic regulation is not merely the product of a single negative feedback cycle but reflects the complex interaction of multiple feedback systems that can be modified by higher control centers. This hierarchical control and feedback redundancy produces both a finer level of control and a greater flexibility that enables the organism to adapt to changing environmental conditions. The health and vitality of the organism can be said to be the end result of homeostatic regulation of the internal environment; an understanding of normal physiology is not possible without an appreciation of this concept. Conversely, it follows that disruption of homeostatic mechanisms is what leads to disease, and effective therapy must be directed toward re-establishing these homeostatic conditions, working with rather than against nature.

Author Contributions

GB prepared all aspects of this review article.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

As previously stated, portions of the material presented in this essay were previously published as Chapter 10 in “Handbook of Systems and Complexity in Health” ( Billman, 2013 ) and are reproduced (with modification and updates) with the permission of the publisher.

1 http://www.brainyquote.com/quotes/quotes/t/tomrobbins404093.html

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Best topics on Homeostasis

1. Biology and Functions of Homeostasis and Thermoregulation

2. Role Homeostasis in Understanding the Human Body

3. Generalised Anxiety Disorder (GAD) and Homeostasis

4. Evaluation of Dynamic Serum Thiol-Disulphide Homeostasis in Colorectal Cancer

5. Endocrine Diseases In Patients Admitted To Endocrinology Ward

6. Renal And Urinary Systems’ Work

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Homeostasis

Homeostasis Definition

“Homeostasis is the state of steady internal chemical and physical conditions maintained by living systems.”

Table of Contents

• Explanation
• Body System

Homeostasis Meaning and Etymology

The theory of homeostasis was first introduced by Claude Bernard, a French Physiologist in the year 1865, and the term was first used in 1926 by Walter Bradford Cannon. Bradford derived Homeostasis from the ancient Greek words  ὅμοιος (pronounced: hómoios) and ἵστημι (pronounced: hístēmi). The combination of these words translates to “similar” and “standing still” respectively.

Read on to explore what is homeostasis and its role in regulating internal body environment.

What is Homeostasis?

Homeostasis is quite crucial for the survival of organisms. It is often seen as a resistance to changes in the external environment. Furthermore, homeostasis is a self-regulating process that regulates internal variables necessary to sustain life.

In other words, homeostasis is a mechanism that maintains a stable internal environment despite the changes present in the external environment.

The body maintains homeostasis by controlling a host of variables ranging from body temperature, blood pH, blood glucose levels to fluid balance, sodium, potassium and calcium ion concentrations.

Regulation of Homeostasis

The regulation of homeostasis depends on three mechanisms:

• Control Center.

The entire process continuously works to maintain homeostasis regulation.

As the name suggests, the receptor is the sensing component responsible for monitoring and responding to changes in the external or internal environment.

Control Center

The control centre is also known as the integration centre. It receives and processes information from the receptor.

The effector responds to the commands of the control centre. It could either oppose or enhance the stimulus.

Also Read:  Thermoregulation

Homeostasis Breakdown

The failure of homeostasis function in an internal environment will result in illnesses or diseases. In severe cases, it can even lead to death and disability.

Many factors can affect homeostasis. The most common are:

• Physical condition.
• Diet and nutrition.
• Venoms and toxins.
• Psychological health.
• Side effects of medicines and medical procedures.

Body Systems and Homeostasis

The body system participates in maintaining homeostasis regulations. The purpose of the body system is to describe several controlling mechanisms where every system contributes to homeostasis.

Listed below are the tables which describe how different organs perform different functions to maintain the internal body environment.

Other Examples of Homeostasis

• Blood glucose homeostasis.
• Blood oxygen content homeostasis.
• Extracellular fluid pH homeostasis.
• Plasma ionized calcium homeostasis.
• Arterial blood pressure homeostasis.
• Core body temperature homeostasis.
• The volume of body water homeostasis.
• Extracellular sodium concentration homeostasis.
• Extracellular potassium concentration homeostasis.
• Blood partial pressure of oxygen and carbon dioxide homeostasis.

Also Read:  Osmoregulation

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Frequently Asked Questions

1. state homeostasis definition..

Homeostasis is the ability to maintain internal stability in an organism in response to the environmental changes. The internal temperature of the human body is the best example of homeostasis.

2. Which body systems help to maintain homeostasis?

The endocrine system and the nervous system are essential in maintaining the homeostasis of the body. However, other organs also play a role in maintaining homeostasis as well.

3. How is homeostasis essential for our body?

Homeostasis is a self-regulating process that controls internal variables necessary to sustain life.

4. What are the main components of homeostasis?

Homeostasis involves three components- the receptor, the control centre, and the effector. The receptor receives information on the changing environment, and the control centre processes the information received by the receptor. And the effector responds to the commands of the control centre by enhancing or opposing the stimulus.

5. What is the primary function of homeostasis?

The primary function of homeostasis is to maintain a balance within the body regarding its temperature, salt concentration, food intake and pH levels.

6. How does the cell maintain homeostasis in the body?

To maintain homeostasis in the body, the cells perform the following activities: Obtain and use energy, exchange materials, make new cells, and eliminate wastes.

7. What role does liver play in homeostasis?

Our liver plays a vital role in blood glucose homeostasis. When the blood glucose level rises after a meal, the liver removes glucose from the blood and stores it in the form of glycogen. When the blood glucose levels are low, it converts the stored glycogen back to glucose.

8. How does the skin help in maintaining homeostasis?

If the external temperature is high, the body tries to keep cool by producing sweat. Also, blood vessels near the skin surface dilate. This helps in decreasing body temperature. Conversely, if the external temperature is cold, the blood vessels constrict and retain body heat. Thus, the skin maintains homeostasis.

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The Importance of Homeostasis Essay

Homeostasis refers to the balance in the external and internal environments of living organisms that enables them to survive within a range of conditions. Through this self-regulating process, changes in the body and the mechanisms that react to these changes can be easily detected to restore stability. Homeostatic regulatory components that maintain equilibrium include effectors, receptors, and control centers (Fossion et al., 2018). The disruption of homeostatic mechanisms results in disease, and therefore, the interaction of multiple complex feedback systems is vital to ensure the health of organisms. Homeostasis is crucial in regulating various concentrations of pH, ions, blood sugar, fluids, and temperature in the body despite variations in the diet and environment to help maintain life.

Receptors primarily sense, monitor, and respond to internal and external changes in the environment for an appropriate reaction to be elicited by the body. The effectors are the body organs and tissues that receive information about the changes and respond by providing the conditions necessary to maintain homeostasis. The control centers, such as the respiratory system, are crucial in setting the maintenance range for particular variables. However, the homeostatic control of an entity does not necessarily mean that its value is absolutely steady in health (Fossion et al., 2018). For instance, as regulated by a homeostatic mechanism with temperature sensors, the setpoint of core body temperature varies from time to time and needs to be reset. Notably, the body temperature in humans and other mammals changes during the course of the day, with the lowest temperatures being at night and the highest in the afternoons.

The nervous system, digestive system, and endocrine system, among other body systems, work together to contribute to overall homeostasis. In mammals, body temperature regulation is achieved through input from thermoreceptors in the hypothalamus, spinal cord, and internal organs (Fossion et al., 2018). When the core temperature drops, the blood supply to the skin and limbs is reduced, resulting in minimal heat loss. On the other hand, when the temperature is high, sweat glands are stimulated to secrete sweat onto the skin, which evaporates, cooling the skin and blood flowing through it.

Homeostasis is also important in controlling the blood sugar levels through the beta cells of the pancreas. In response to high sugar levels, insulin is secreted into the blood, which inhibits the secretion of glucagon from alpha cells and glucose from the liver. With the limited secretion of more glucose into the blood, fat cells and muscle cells take up and convert the excess glucose to other forms (Fossion et al., 2018). However, when the level of blood glucose falls, insulin secretion is stopped, and glucagon is released into the blood thus correcting the detected error.

In regulating the level of blood gases, carbon dioxide and oxygen levels are monitored by various chemoreceptors, and information is relayed to the respiratory center to activate effector organs (Fossion et al., 2018). The diaphragm and other muscles of respiration respond appropriately to variations of gases especially oxygen which serves many purposes in the body. Moreover, the process of homeostasis enables fluid balance as it balances the amount of water and the levels of electrolytes in the body through osmoregulation. When the water levels are low, the kidney has to reabsorb water, thus preventing water loss through urine, and a thirst reflex is generated to restore the required amount of water.

Conclusively, when the body has a suitable constant internal temperature, metabolic processes can effectively take place to release the energy required by an organism to carry out various activities. All the body systems and organs have to work together for the correct signals and responses to take place. Therefore, homeostatic regulation is a necessary process in maintaining the immunity and proper functioning of the body.

Fossion, R., Rivera, A., & Estanol B. (2018). A physicist’s view of homeostasis: How time series of continuous monitoring reflect the function of physiological variables in regulatory mechanisms. Physiological Measurement, 39 (8), 5-16.

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Role of Homeostasis in Human Physiology

Checked : Suzanne S. , Vallary O.

Latest Update 20 Jan, 2024

Table of content

Levels of action

Systems involved in homeostasis, the feedback mechanism.

Homeostasis is the state of living beings to maintain around a predetermined level the value of some internal parameters, disturbed continuously by various external and internal factors. The ordered set of subsystems that make up the human organism has a network of control systems, whose simultaneous intervention regulates the flow of energy and metabolites. It keeps the internal environment unchanged or almost unchanged, regardless of the modifications of the external one. Self-regulation of living organisms is a fundamental concept of modern biology, formulated at the end of the 19th century. Here, we will discuss the role of homeostasis.

Self-regulation mechanisms operate at all levels of system organization. At the molecular level, feedback inhibition (or feedback) limits the number of final products that are formed by the action of an   enzymatic system . At the cellular level, the phenomenon of contact inhibition intervenes, for which in a reproduction of cells, the process of mitosis stops when they become so numerous that they touch each other. The close physical relationship would allow an inhibiting chemical messenger to pass from one cellular element to another to prevent further division. In tumors, this homeostatic mechanism is lost, and this explains the unstoppable reproduction of neoplastic cellular elements.

At an organismic level, the various mechanisms operate in different ways; the hormonal synthesis activity of the endocrine glands is governed by the events that occur in the systems regulated by the hormones. For instance, the increase in blood sugar stimulates the secretion of insulin, which in turn increases the glucose, with a consequent decrease in its blood concentration. Hunger and thirst are also sensations aimed at maintaining optimal levels of energy, nutrients, and water. Also, at the reproduction level, an example of homeostatic regulation is provided by the relationship between predators and prey.

The optimal functioning of each control system takes place only in a specific area, and its adaptability is therefore limited. However, within any sphere of control, a disturbance can exceed the compensation capacity of a given system, altering the transfer of energy and metabolites through that subsystem. If other control systems are able to compensate for the insufficient one, the stability of the organism is maintained, but with the loss of a part of reserve energy. On the contrary, the other systems cannot exercise this substitute; the entire network becomes unstable and insufficient to ensure control.

All the systems of the organism contribute to the maintenance of the, but the main control center is represented by the central nervous system which determines the most appropriate type of response (endocrine, immune, etc.). Particularly the role of the endocrine system is important because it controls and regulates the other systems of the organism. However, its response is slow (minutes, hours, days). Unlike it is implemented by the nervous system, which instead reacts promptly (fractions of a second or seconds), but whose effects are short-lived. Therefore, cooperation between the two systems provides complementary control methods.

The stimuli represented by the modifications of the external and internal environment are recognized and conveyed, through the afferent nerves to the spinal cord and brain which analyze them, associate them and compare them using an integration process.

A feedback mechanism makes a further modification of a troubled system. Most control systems use negative feedback, other positive feedback. The first consists of compensatory changes that bring the system back to its previous state, thus canceling or limiting the effects of the disturbances. Therefore, it opposes the changes and tends to maintain stability. Negative feedback systems are inherently unstable but are commonly found in endocrine and metabolic regulation. Some of the most typical examples are represented by the control of body temperature and weight. With positive feedback, however, there is a further increase in noise, which, however, allows us to carry out processes that are inactive in rest conditions, amplifying the starting signal (cascade mechanism). Examples are blood coagulation and glycolysis, which constitute self-limiting processes because the availability of substrates (fibrinogen and other coagulation factors, glycogen) is limited.

Sometimes, for a single variable (such as, for example, blood pressure and temperature), there are multiple control systems, which serve to guarantee the maintenance of a certain level even when not all the receptors and effectors of which they consist they are functional. This redundancy, therefore, represents a greater guarantee of control—a series of other baroreceptors located at different levels in the circulatory system. The atria and large vessels can send information to the brain.

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Redundancy can also occur concerning effector mechanisms. Referring once again to the control of   blood pressure , this can occur through changes in both peripheral resistance (contraction or relaxation of the vascular musculature), and in cardiac output (increase or decrease in heart rate or contraction energy). The effect of the abolition of one of these mechanisms is, therefore only transitory, because the other exerts a substitute. Redundancy guarantees the stability of the variable, despite many perturbations of opposite sign. The limits within which specific variables (temperature, blood sugar, blood pressure, bodyweight, etc.) are controllable by the negative feedback processes are usually set point). The oscillations around this point depend on the delay (phase shift) between the recognition, by the receptors, of the modification, and the response of the negative feedback system to that modification.

The frequency and depth of breathing in response to an increase in the partial pressure of CO2   in the arterial blood can vary rapidly due to the information that reaches the brain. They send the information to the brain via the vagus nerve and glossopharyngeal nerves from the peripheral chemoreceptors of the aortic arch and through the stimulation of the central chemoreceptors. Almost all physiological processes are regulated, at all levels, by these control systems. They allow the living being to adapt its biological individuality to preserve its constants against the stresses of the environment. Pathological changes occur when the stimulus is excessive and /or the response is not suitable to satisfy this need for balance and stability of the organism.

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Essay on Homeostasis

Homeostasis is the tendency to maintain a stable and relatively constant internal environment. It is crucial for any living thing to maintain a stable internal condition since it must always remain constant. Despite the external environment’s dynamicity, the body employs different physiological strategies that support the system’s proper function. This capability is one of the crucial aspects that enable the human body to stay alive. The body acts upon and resists the effects of external factors to prevent its deviation from the state of balance, equilibrium, and stability it favors rather than doing nothing. According to Modell et al. (2015), three general components enable the human body to maintain homeostasis. They include the receptor, control, and effector centers.

Maintaining homeostasis

The hemostasis mechanism is in the form of a loop that can either be positive or negative. Positive feedback propels the situation and results in more stimulation, whereas negative feedback decelerates the process and inhibits the stimulus (Castanho & Dos Anjos Garnes, 2019). For example, a high body temperature triggers the negative loop, which returns it towards the set point. The sensors, which are primary nerve cells with endings in the brain, will detect the high temperature and relay it to the temperature-regulatory control center. Processing this information will take part in the control center, and effectors such as sweat glands will be activated (Modell et al., 2015). The function of these effectors is to lower the body temperature by opposing the stimulus.

The body temperature does not always go above the setpoint. In some situations, it can go below the set point. In general, there are at least two negative feedback loops that are usually involved in the homeostatic circuit. The first negative feedback loop is designed to lower a parameter after it has gone above the setpoint (Modell et al., 2015). The second negative feedback loop is intended to return the parameter up when it is below the set point.

For example, when the external body temperature is either too cold or too hot, the hypothalamus, the temperature regulatory center in the brain, is notified by the sensors in the periphery that the temperature has strayed from the setpoint (Tansey et al., 2015). For instance, when an individual has been exercising too hard, the internal body temperature rises above the setpoints, and cooling it down will require activating the necessary mechanisms (Library, 2019). The increase in blood flow in the skin seeks to increase heat loss to the surrounding. Sweating is also a mechanism that allows the body to cool off when the sweat evaporates.

The temperature center in the brain will also trigger responses to keep warm when an individual is sitting in a cold room and is not dressed warmly. Tansey et al. (2015) note that a person may begin shivering since the blood flow in the skin decreases. This action allows the body to generate more heat. The skin may also develop goosebumps, allowing the body hair to stand up and trap air near the skin.

The negative feedback loops play a fundamental factor in homeostasis, and any interference with this feedback mechanism disrupts homeostasis and may eventually result in disease. For example, a broken feedback loop involving insulin hormone results in diabetes disease (Röder et al., 2016). The human body finds it challenging to lower high blood sugar levels when the feedback loop is broken. When an individual consumes a meal, the blood glucose levels increase, triggering the β cells in the pancreas to secrete insulin (Library, 2019). It then activates body cells to absorb this glucose for energy. Insulin is also responsible for the conversion of glucose to glycogen in the liver. These processes are responsible for reducing glucose levels in the blood, which returns the system to homeostasis.

On the other hand, glucagon increases glucose concentration in the blood. When the blood glucose levels are low, the pancreatic α cells release glucagon, which initiates the breakdown of glycogen to glucose in the liver (Röder et al., 2016). This increases the glucose level in the body. The system is brought back to homeostasis when glucagon secretion is reduced. Therefore, diabetes occurs when the human body stops responding to insulin or when the pancreas fails to produce enough insulin. Therefore, blood sugar remains high under these conditions since the body cells cannot absorb glucose readily.

Ecosystem homeostasis

Ecosystems comprise a network of animals from the tiniest insects to the largest mammals, alongside various microorganisms, fungi, and plants, making the ecosystem complex. There is an interaction between all these lifeforms since caterpillars will feed on leaves, bears prey on fish, while shrews eat insects. A delicate balance is maintained by everything that exists in nature, and scientists refer to the balance of organisms in an ecosystem as ecosystem homeostasis (Zakharov et al., 2018). The fundamental goal of ecosystem homeostasis is equilibrium. But nothing is ever perfectly balanced in the real-world ecosystem. Various animal species have their population at a similar range, resulting in a relatively stable state of an ecosystem in equilibrium (Ecological Center, 2021). Therefore, as long as there is no general downward or upward trend, populations can go up and down in cycles.

Negative feedback in ecosystem homeostasis operates more diffusely than physiology due to the decentralized nature of ecological systems such as communities, populations, and ecosystems. The interactions among species, individuals, and their environment result in negative feedback since a central processing unit that implements and coordinates negative feedback is unavailable (Ecological Center, 2021). A classic example of how negative feedback could stabilize the system due to consumer or resource dynamics is the interaction between predators and prey. There is an increase in resource availability for the predators when the prey population increases. This increase results in increased survival and reproduction rates for the predator due to the consumption of the prey. Hence, the predator population increases. However, an increase in predator population increases the demand for prey, increasing the predators’ death rates. This is due to the decline in the prey population, which cannot support the high predator abundance. Eventually, the predator population also reduces. Therefore, the limiting resource of the prey induces negative feedback, which counteracts the initial increase in predator abundance.

Many systems experience such a case, and there is a strong stabilizing constraint on the community’s dynamic when food resources are limiting. An increase in resource consumption results in a decrease in other components since the resources are limited. The Ecological Center (2021) defines compensatory dynamics or species compensation as the balance between increased and decreased species. The negative feedback that counteracts the increased consumption rates by various communities emerges from the finite nature of these resources. Resources are responsible for reproduction, growth, maintenance, and survival and as a result, limiting them affects net production, birth, and death rates. Therefore, resource constraints are essential for stabilizing the overall consumption and stabilizing critical ecosystem properties such as biomass production, total abundance, and standing biomass.

In conclusion, people have been steadily growing, particularly since the industrial revolution. It took several years for the human population to reach one billion, and the world is now swiftly approaching eight billion, more than three hundred years later. This sounds like the world is on its way to a catastrophe due to overpopulation. However, due to decreased birth rates resulting from increasing access to contraception and women’s education, the world’s population is not predicted to grow exponentially. The average family size in countries where women are empowered is tiny, with very few children. An increase in natural and economic resource demand and competitiveness has led to reduced birth rates, leading to a drop in the world population.

Castanho, F. L., & Dos Anjos Garnes, S. (2019).  Homeostasis: An Integrated Vision . IntechOpen. https://books.google.co.ke/books?id=y7-QDwAAQBAJ

Ecological Center. (2021).  Maintenance of Homeostasis in Ecological Systems – Population Dynamics . https://www.ecologycenter.us/population-dynamics-2/maintenance-of-homeostasis-in-ecological-systems.html

Library, T. O. T. O. C. (2019).  The Animal Body – Basic Form and Function: Biology . Independently Published. https://books.google.co.ke/books?id=IPNMzQEACAAJ

Modell, H., Cliff, W., Michael, J., McFarland, J., Pat Wenderoth, M., & Wright, A. (2015). A Personal View A physiologist’s view of homeostasis.  Adv Physiol Educ ,  39 , 259–266. https://doi.org/10.1152/advan.00107.2015.-Homeostasis

Röder, P. V., Wu, B., Liu, Y., & Han, W. (2016). Pancreatic regulation of glucose homeostasis.  Experimental & Molecular Medicine ,  48 (November 2015), e219. https://doi.org/10.1038/emm.2016.6

Tansey, E. A., Johnson, C. D., & Johnson, C. D. (2015). Staying Current Recent advances in thermoregulation.  Adv Physiol Educ ,  39 , 139–148. https://doi.org/10.1152/advan.00126.2014.-Ther

Zakharov, V., Minin, A., & Trofimov, I. (2018). Study of developmental homeostasis: From population developmental biology and the health of environment concept to the sustainable development concept.  Russian Journal of Developmental Biology ,  49 (1). https://doi.org/10.1134/S1062360418010071

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Homeostasis Of Blood Pressure

• Category Health
• Subcategory Human Body
• Topic Homeostasis

Homeostasis defines a self-regulating system within the body that maintains a steady balance while adjusting to conditions that are crucial for the body to keep functioning efficiently (The Editors of Encyclopedia Britannica). This essay will discuss homeostasis, the homeostatic mechanism controlling blood pressure and the consequences of the homeostatic imbalance.

Blood pressure measures the amount of pressure your blood is having on the wall of your arteries throughout your body. Blood pressure plays a crucial part in how your heart and circulation works within your body. If it is not controlled within the ideal ranges (also known as the set point) and homeostasis is not achieved it can result in either hypertension (high blood pressure) or hypotension (low blood pressure). Both of which can result in severe consequences such as heart attacks, stroke, kidney disease, and even fatality (Marieb & Hoehn, 2019).

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The homeostasis of blood pressure works by using a negative feedback loop (Figure, 1) that is made up of three components: the sensor, control centre and the effector. The sensor, which is also known as the baroreceptor, monitors the physical values. The control centre constantly compares the ideal range. If the range moves too far away from the set point the control centre will then activate an effector (Marieb & Hoehn, 2019; Top Hat, 2019).Figure 1 (TopHat, 2019)

Baroreceptors, which are located in the carotid sinus and the aortic arch are the sensors that are constantly measuring blood pressure within the body. It is made up of nerve fibres that extend within the walls of the arterial vessels. (Physiology Illustrated, 2005) When changes outside of the ideal range are detected it stimulates the nerve endings within in the arterial walls, sending electrical signals firing in the baroceptor neurons. These signals travel to the cardiovascular control centre in the brain to initiate the baroreflex to regulate the blood pressure (Huang, Y., Jiang, L., Lau, O-C., Lo, Y-C., Mak, F-T,A., Yao, X. & Yao, Y.; Figure 2, Marieb & Hoehn, 2019; Rabinovitch, A., Friedman, M., Braunstein, D., Biton, Y., & Aviram, I.).

In hypertension the effectors, the heart and blood vessels will aim to restore the balance with the use of two mechanisms, vasodilation and decreased cardiac output. With the use of these two mechanisms the heart rate will decrease and the vessels will expand in diameter, causing the blood pressure to drop back within the ideal range (Marieb & Hoehn, 2019; Scogna, 2014; Top Hat, 2019).

In hypotension the effector will aim to restore the balance via vasoconstriction and increased output. The use of these mechanisms will increase the heart rate and decrease the diameter of the blood vessels returning the value back to the set point to avoid homeostatic imbalance (Marieb & Hoehn, 2019; Scogna, 2014; Top Hat, 2019). Figure 2 (Marieb & Hoehn, 2019)

Sometimes the set point can be reset under particular conditions such as exercise, when the blood pressure usually increases with exertion. This particular increase is not considered abnormal, it is the body’s response to the heightened need for oxygen within the muscle tissues. When the muscles require more oxygen, the body reacts by increasing the blood flow to muscle tissues, which increases blood pressure. This resetting of the normal homeostatic set point is needed to meet the heightened need for oxygen by muscles (Scogna, 2014).

The homeostatic imbalance of blood pressure can have several consequences to other body systems, such as the renal and cardiovascular systems. For example; hypertension can lead to renal failure. When hypertension is apparent for a long period of time it damages the blood vessels causing obstruction of blood flow to the kidneys, which then causes the filtration process to slow down or in some cases, completely stop (Marieb & Hoehn, 2019).

Hypotension will result from increased strain on the heart, impairing its ability to maintain normal cardiac output, resulting in shock. Low cardiac output results in poor blood flow to organs which leads to organ failure as there is not enough blood flow to meet the needs of body tissues. Inadequate supply of blood to the organ cells causes oxygen starvation which results in cell death (Bidani & Griffen, 2014; Marieb & Hoehn, 2019).

The homeostasis of blood pressure includes both short and long term controls. With the short term control being the hormonal control and the baroreflex (neural control). The renal mechanisms act as the body’s long term regulator of blood pressure. All these controls work together to maintain blood pressure within the normal range. I have highlighted the action of the neural control which only makes up a component of the homeostasis of blood pressure (Marieb & Hoehn, 2019).

• Bidani, A. K. , & Griffin, K. A. (2004). Pathophysiology of hypertensive renal damage: Implications for therapy. Hypertension, 44(5), 595-601. doi:10.1161/01.HYP.0000145180.38707.84
• [bookmark: _Hlk18602868]Huang, Y., Jiang, L., Lau, O-C., Lo, Y-C., Mak, F-T,A., Yao, X. & Yao, Y., Frontiers in Physiology. (2016). Vol 7:384, Aortic baroreceptors display higher mechanosensitivity than carotid baroreceptors. doi:10.3389/fphys.2016.00384/full
• Marieb, E.N, & Hoehn K. (2019). Human anatomy & physiology (11th ed). Essex, England: Pearson Education
• [bookmark: _Hlk18574717]Rabinovitch, A., Friedman, M., Braunstein, D., Biton, Y., & Aviram, I. (2015). The baroreflex mechanism revisited. Bulletin of Mathematical Biology, 77(8), 1521-1538. doi:10.1007/s11538-015-0094-
• Scogna, K. (2014) Homeostasis. In K. L.Lerner & B.W.Lerner (Eds). The gale encyclopedia of science, (5th ed), 2014, Retrieved from https://link-gale-com.elibrary.jcu.edu.au/apps/doc/CX3727801229/ITOF?u=james_cook&sid=ITOF&xid=5c5bd8b9
• The Editors of Encyclopedia Britannica. (n.d). Homeostasis biology. retrieved from https://www.britannica.com/science/homeostasis
• Top Hat. (2019). Introduction to health science: A top hat interactive text. Toronto, Canada: Top Hat Monocle.

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Home — Essay Samples — Nursing & Health — Homeostasis — Homeostasis Movie Reflection

Homeostasis Movie Reflection

• Categories: Homeostasis

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Words: 712 |

Published: Mar 20, 2024

Words: 712 | Pages: 2 | 4 min read

Table of contents

Introduction, portrayal of homeostasis in "inside out", relevance to biological homeostasis, reflection on the movie's message.

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