Homeostasis is the property of either an open system or a closed system, especially a living organism, which regulates its internal environment so as to maintain a stable, constant condition. Multiple dynamic equilibrium adjustments, regulation mechanisms, make homeostasis possible. The term was coined in 1932 by Walter Bradford Cannon from the Greek homoios (same, like, resembling) and stasis (to stand, posture).
- 1 Biological homeostasis
- 2 Ecological homeostasis
- 3 Reactive homeostasis
- 4 Other fields
- 5 See also
- 6 References
The homeostatic maintenance of internal bodily conditions within tolerable limits is one fundamental characteristic of living things. In order for an organism's life systems to function properly, the tissues and cells require appropriate conditions. Homeostasis depends on the dynamic action and interaction of a number of body systems. Factors such as temperature, salinity, acidity, plus nutrient and waste balances all affect a complex organism's ability to sustain life.
With regard to any given life system parameter, an organism may be a conformer or a regulator. Regulators try to maintain the parameter at a constant level over possibly wide ambient environmental variations. On the other hand, conformers allow the environment to determine the parameter. For instance, endothermic animals maintain a constant body temperature, while ectothermic animals exhibit wide body temperature variation.
This is not to say that conformers don't have behavioural adaptations allowing them to exert some control over a given parameter. For instance, reptiles often rest on sun-heated rocks in the morning to raise their body temperature. Likewise, regulators' behaviors may contribute to their internal stability: The same sun-baked rock may host a ground squirrel, also basking in the morning sun.
An advantage of homeostatic regulation is that it allows an organism to function effectively in a broad range of environmental conditions. For example, ectotherms tend to become sluggish at low temperatures, while a co-located endotherm may be fully active. That thermal stability comes at a price since an automatic regulation system requires additional energy. One reason snakes may eat only once a week is that they use much less energy to maintain homeostasis.
All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: The receptor is the sensing component that monitors and responds to changes in the environment. When the receptor senses a stimulus, it sends information to a control center, the component that sets the range at which a variable is maintained. The control center determines an appropriate response to the stimulus. The result of that response feeds to the receptor, either enhancing it with positive feedback or depressing it with negative feedback 
Negative Feedback Mechanisms
Negative feedback mechanisms reduce or suppress the original stimulus, given the effector’s output. Most homeostatic control mechanisms require a negative feedback loop to keep conditions from exceeding tolerable limits. The purpose is to prevent sudden severe changes within a complex organism. There are hundreds of negative feedback mechanisms in the human body. Among the most important regulatory functions are: thermoregulation, osmoregulation, and glucoregulation. The kidneys contribute to homeostasis in four important ways: regulation of blood water levels, reabsorption of substances into the blood, maintenance of salt and ion levels in the blood, and excretion of urea and other wastes.
A negative feedback mechanism example is the typical home heating system. Its thermostat houses a thermometer, the receptor that senses when the temperature is too low. The control center, also housed in the thermostat, senses and responds to the thermometer when the temperature drops below a specified set point. Below that target level, the thermostat sends a message to the effector, the furnace. The furnace then produces heat, which warms the house. Once the thermostat senses a target level of heat has been reached, it will signal the furnace to turn off, thus maintaining a comfortable temperature - not too hot nor cold. 
Positive Feedback Mechanisms
Positive feedback mechanisms are designed to accelerate or enhance the output created by a stimulus that has already been activated. Unlike negative feedback mechanisms that initiate to maintain or regulate physiological functions within a set and narrow range, the positive feedback mechanisms are designed to push levels out of normal ranges. To achieve this purpose, a series of events initiates a cascading process that builds to increase the effect of the stimulus. This process can be beneficial but is rarely used by the body due to risks of the acceleration becoming uncontrollable.
One bodily positive feedback example event is blood platelet accumulation which in turn causes blood clotting in response to a break or tear in the lining of blood vessels. Another example is the release of oxytocin to intensify the contractions that take place during childbirth.
Positive feedback can also be harmful. An example being when you have a fever it causes a positive feedback within homeostasis that pushes the temperature continually higher. Body temperature can reach extremes of 45ºC (113ºF), at which cellular proteins denature, causing the active site in proteins to change, thus causing metabolism stop and ultimately resulting in death.
Much disease results from disturbance of homeostasis, a condition known as homeostatic imbalance. As it ages, every organism will lose efficiency in its control systems. The inefficiencies gradually result in an unstable internal environment that increases the risk for illness. In addition, homeostatic imbalance is also responsible for the physical changes associated with aging. Even more serious than illness and other characteristics of aging, is death. Heart failure has been seen where nominal negative feedback mechanisms become overwhelmed, and destructive positive feedback mechanisms then take over.
Varieties of homeostasis
The Dynamic Energy Budget theory for metabolic organisation delineates structure and (one or more) reserves in an organism. Its formulation is based on three forms of homeostasis:
- Strong homeostasis is where structure and reserve do not change in composition. Since the amount of reserve and structure can vary, this allows a particular change in the composition of the whole body (as explained by the Dynamic Energy Budget theory).
- Weak homeostasis is where the ratio of the amounts of reserve and structure becomes constant as long as food availability is constant, even when the organism grows. This means that the whole body composition is constant during growth in constant environments.
- Structural homeostasis means that the sub-individual structures grow in harmony with the whole individual; the relative proportions of the individuals remain constant.
In disturbed ecosystems or sub-climax biological communities such as the island of Krakatoa, after its major eruption in 1883, the established stable homeostasis of the previous forest climax ecosystem was destroyed and all life eliminated from the island. In the years after the eruption, Krakatoa went through a sequence of ecological changes in which successive groups of new plant or animal species followed one another, leading to increasing biodiversity and eventually culminating in a re-established climax community. This ecological succession on Krakatoa occurred in a number of several stages, in which a sere is defined as "a stage in a sequence of events by which succession occurs". The complete chain of seres leading to a climax is called a prisere. In the case of Krakatoa, the island as reached its climax community with eight hundred different species being recorded in 1983, one hundred years after the eruption which cleared all life off the island. Evidence confirms that this number has been homeostatic for some time, with the introduction of new species rapidly leading to elimination of old ones.
The evidence of Krakatoa, and other disturbed or virgin ecosystems shows that the initial colonisation by pioneer or R strategy species occurs through positive feedback reproduction strategies, where species are weeds, producing huge numbers of possible offspring, but investing little in the success of any one. Rapid boom and bust plague or pest cycles are observed with such species. As an ecosystem starts to approach climax these species get replaced by more sophisticated climax species which through negative feedback, adapt themselves to specific environmental conditions. These species, closely controlled by carrying capacity, follow K strategies where species produce fewer numbers of potential offspring, but invest more heavily in securing the reproductive success of each one to the micro-environmental conditions of its specific ecological niche.
It begins with a pioneer community and ends with a climax community. This climax community occurs when the ultimate vegetation has become in equilibrium with the local environment.
Such ecosystems form nested communities or heterarchies, in which homeostasis at one level, contributes to homeostatic processes at another holonic level. For example, the loss of leaves on a mature rainforest tree gives a space for new growth, and contributes to the plant litter and soil humus build-up upon which such growth depends. Equally a mature rainforest tree reduces the sunlight falling on the forest floor and helps prevent invasion by other species. But trees too fall to the forest floor and a healthy forest glade is dependent upon a constant rate of forest regrowth, produced by the fall of logs, and the recycling of forest nutrients through the respiration of termites and other insect, fungal and bacterial decomposers. Similarly such forest glades contribute ecological services, such as the regulation of microclimates or of the hydrological cycle for an ecosystem, and a number of different ecosystems act together to maintain homeostasis perhaps of a number of river catchments within a bioregion. A diversity of bioregions similarly makes up a stable homeostatic biological region or biome.
In the Gaia hypothesis, James Lovelock stated that the entire mass of living matter on Earth (or any planet with life) functions as a vast homeostatic superorganism that actively modifies its planetary environment to produce the environmental conditions necessary for its own survival. In this view, the entire planet maintains homeostasis. Whether this sort of system is present on Earth is still open to debate. However, some relatively simple homeostatic mechanisms are generally accepted. For example, when atmospheric carbon dioxide levels rise, certain plants are able to grow better and thus act to remove more carbon dioxide from the atmosphere. When sunlight is plentiful and atmospheric temperature climbs, the phytoplankton of the ocean surface waters thrive and produce more dimethyl sulfide, DMS. The DMS molecules act as cloud condensation nuclei which produce more clouds and thus increase the atmospheric albedo and this feeds back to lower the temperature of the atmosphere. As scientists discover more about Gaia, vast numbers of positive and negative feedback loops are being discovered, that together maintain a metastable condition, sometimes within very broad range of environmental conditions.
Example of use: "Reactive homeostasis is an immediate response to a homeostatic challenge such as predation."
However, any homeostasis is impossible without reaction - because homeostasis is and must be a "feedback" phenomenon.
The phrase "reactive homeostasis" is simply short for: "reactive compensation reestablishing homeostasis", that is to say, "reestablishing a point of homeostasis." - it should not be confused with a separate kind of homeostasis or a distinct phenomenon from homeostasis, it is simply the compensation (or compensatory) phase of homeostasis.
The term has come to be used in other fields, as well.
An actuary may refer to risk homeostasis, where (for example) people who have anti-lock brakes have no better safety record than those without anti-lock brakes, because they unconsciously compensate for the safer vehicle via less-safe driving habits. Previously, certain maneuvers involved minor skids, evoking fear and avoidance: now the anti-lock system moves the boundary for such feedback, and behavior patterns expand into the no-longer punitive area. It has also been suggested that ecological crises are an instance of risk homeostasis in which behavior known to be dangerous continues until dramatic consequences actually occur.
Sociologists and psychologists may refer to stress homeostasis, the tendency of a population or an individual to stay at a certain level of stress, often generating artificial stresses if the "natural" level of stress is not enough.
Jean Francois Lyotard, a postmodern theorist, has applied this term to societal 'power centers' that he describes as being 'governed by a principle of homeostasis.' For example the scientific hierarchy, which will sometimes ignore a radical new discovery for years because it destabilizes previously accepted norms. (See "The Postmodern Condition: A Report on Knowledge" by J.F. Lyotard)
A 2007 study purported to find (and show clinically) conversational homeostasis in which overly-familiar people (such as spouses) condense their speech so much that they are actually worse at communicating novel information than strangers are, while not being conscious of this problem. 
- Farabee.Michael J.Ph.D. (2006)"Animal Organ Systems and Homeostasis" Estrell Mountain Community Collegehttp://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookANIMORGSYS.html
- Marieb, Elaine N. & Hoehn, Katja (2007). Human Anatomy & Physiology (Seventh ed.). San Francisco, CA: Pearson Benjamin Cummings.
- Potter, Andrew (2007), "Planet-friendly design? Bah, humbug.", MacLean's 120(5): 14
- Keysar, Boaz (2007), "The Effect of Information Overlap on Communication Effectiveness", Cognitive Science
- Winston, David & Maimes, Steven. “ADAPTOGENS: Herbs for Strength, Stamina, and Stress Relief,” Healing Arts Press, 2007.
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