Definition: Homeostasis

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A built-in, automated property of a system that executes and monitors events essential to the existence of the system, such as animal breathing and instinct. It is a self-regulating mechanism that allows a system to avoid paying detailed attention to its most basic functions thereby helping keep it in a steady state.(Koestler, 1967)

Impaired Homeostasis

According to Stedman's Medical Dictionary, homeostasis refers to: 1. The state of equilibrium in the body with respect to various functions and to the chemical compositions of the fluids and tissues. 2. The process through which such bodily equilibrium is maintained. Hence impaired homeostasis refers to states of imbalance. Important concepts under impaired homeostasis in this program focus on normal aging vs pathological processes.

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HOMEOSTASIS {hohm-ee-oh-stay'-sis}

Homeostasis is the maintenance of equilibrium, or constant conditions, in a biological system by means of automatic mechanisms that counteract influences tending toward disequilibrium. The development of the concept, which is one of the most fund mental in modern biology, began in the 19th century when the French physiologist Claude BERNARD noted the constancy of chemical composition and physical properties of blood and other body fluids. He claimed that this "fixity of the milieu interieur" was essential to the life of higher organisms. The term homeostasis was coined by the 20th-century American physiologist Walter B. Cannon, who refined and extended the concept of self-regulating mechanisms in living systems.

Homeostatic mechanisms operate at all levels of organization in living systems, including the molecular, cellular, organismic, and even populational levels. In complex organisms, such as humans, it involves constant monitoring and regulating of numerous factors, including the gases oxygen and carbon dioxide, nutrients, hormones, and organic and inorganic substances. The concentrations of these substances in body fluid remain unchanged, within limits, despite changes in the external environment.

At the molecular level, a homeostatic mechanism called feedback inhibition operates to limit the amount of chemical product produced by an enzyme system. An enzyme system consists of several enzymes that act sequentially to convert a metabolite into an end product which the organism needs. Overproduction of the end product is prevented by the inhibitory effect of the end product on the first enzyme in the sequence, the regulatory enzyme. As the end product is used up in subsequent metabolic conversions, however, its inhibitory effect on the regulatory enzyme decreases, so that more end product can be formed by the enzyme system. In this manner, the level of end product is maintained at a fairly constant level.

An example of homeostasis in cells is the phenomenon called contact inhibition, in which division in a population of cells stops when they become so numerous that they touch each other. It is believed that, a chemical "messenger" that inhibits further cell division is passed from cell to cell. In contrast, cultured, or artificially produced, cancer cells continue dividing even after cells touch. Thus, cancer cells appear to have lost the homeostatic mechanism of contact inhibition. Homeostasis in organisms is exemplified by the operations of the endocrine system. The hormone-synthesizing activities of the endocrine glands are regulated by events occurring in the systems that the hormones regulate. For example, a rise of blood-glucose levels stimulates the pancreas to secrete insulin, which acts to accelerate the removal of glucose from the blood by conversion into the storage products glycogen and fat. The sensations of hunger and thirst are also homeostatic mechanisms; they help the organism maintain optimum levels of energy, nutrients, and water.

Homeostatic mechanisms also operate to regulate the size of populations. An example is the relationship between the populations of predatory animals and their prey. If prey become abundant, so do their predators, until predation diminishes the supply of prey and causes a decline in the predator population. This allows the prey population to build up again, and the cycle is repeated. In this manner, the populations of both kinds of animals oscillate around a mean.

Peter L. Petrakis

Pribor, Donald B., Functional Homeostasis (1986)
Trojan, P., Ecosystem Homeostasis (1984).

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Based on Information from School of Biochemistry at LaTrobe University:

What is homeostasis?
How is homeostasis achieved?
Feedback mechanisms
Feedback loops


Homeostasis is the maintenance of a constant internal environment (the immediate surroundings of cells) in response to changes in:

the changing conditions of the external environment.
the changing conditions of the internal environment.

Homeostasis is a self adjusting mechanism involving feedback where the response to a stimulus alters the internal conditions and may itself become a new stimulus.

Homeostasis works to maintain the organism's internal environment within tolerance limits - the narrow range of conditions where cellular processes are able to function at a level consistent with the continuation of life.


To maintain cells, tissues and entire organisms within their biological tolerance limits, various mechanisms have evolved. These may be

structural: the animal or plant has particular physical features which help its survival in an otherwise hostile environment.

functional: the metabolism of the animal or plant is able to adjust to changes in conditions as they are detected.

behavioral: the actions and interactions of the individual, either alone or with others, help it to survive in its particular environment.

Homeostasis is really the combined result of all of these, a failure of any one of them can result in the death of an individual.


Feedback mechanisms are the general mechanism of nervous or hormonal regulation in animals. Essentially, feedback occurs when the response to a stimulus has an effect of some kind on the original stimulus. The nature of the response determines how the feedback is 'labeled'.

Negative feedback is when the response diminishes the original stimulus. Positive feedback is when the response enhances the original stimulus.

Negative feedback is most common in biological systems. Examples of this are:

Blood glucose concentrations rise after a sugary meal (the stimulus), the hormone insulin is released and it speeds up the transport of glucose out of the blood and into selected tissues (the response), so blood glucose concentrations decrease (thus decreasing the original stimulus).

Exercise creates metabolic heat which raises the body temperature (the stimulus), cooling mechanisms such as vasodilation (flushed skin) and sweating begin (the response), body temperature falls (thus decreasing the original stimulus).

Positive feedback is less common, which is understandable, as most changes to steady state pose a threat, and to enhance them would be most unhelpful. However, there are a few examples:

A baby begins to suckle her mother's nipple and a few drops of milk are released (the stimulus). This encourages the baby and releases a hormone in the mother which further stimulates the release of milk (the response). The hungry baby continues to suckle, stimulating more milk release until she stops. (Positive feedback, it would not have helped the baby if suckling decreased milk flow, as in negative feedback!)

A ripening apple releases the volatile plant hormone ethylene (the stimulus). Ethylene accelerates the ripening of unripe fruit in its vicinity so nearby fruit also ripens, releasing more ethylene (the response). All the fruit quickly becomes ripe together. ("One 'bad' apple has ruined the whole lot." The biological explanation - positive feedback - for an old saying!)


Regardless of whether the feedback is positive or negative, feedback mechanisms have certain essential components.

Students should be able to identify each of these and explain their role.

Stimulus: The change from ideal or resting conditions.

Receptor: The cells or tissue which detects the change due to the stimulus.

Relay: The transmission of the message, via nerves or hormones or both, to the effector.

Effector: The cells or tissue, usually a gland or muscles, which cause the response to happen.

Response: An action, at cell, tissue or whole organism level which would not have occurred in the absence of the stimulus.

Feedback: The consequence of the response on the stimulus. May be positive or negative.

These feedback loops, as they are often called, are usually well illustrated in textbooks.

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