Homeostasis is the collection of processes by which living systems maintain relatively stable internal conditions despite disturbances originating inside or outside them. A central concept in physiology, it concerns the regulation of variables such as body temperature, fluid composition, acidity, and nutrient availability. Stability does not mean complete constancy: regulated variables fluctuate within ranges, while the mechanisms maintaining them continually adjust their activity. Homeostasis therefore describes an actively maintained condition rather than an unchanging state. (openstax.org)
Historical development
The concept developed from the nineteenth-century work of French physiologist Claude Bernard. Bernard emphasized the milieu intérieur, or internal environment surrounding an organism’s cells, whose relative constancy allows their functioning to remain partly independent of external conditions. American physiologist Walter Bradford Cannon introduced the term homeostasis in 1926 and popularized it through The Wisdom of the Body in 1932. His formulation emphasized coordinated physiological processes rather than stability arising by chance. (pmc.ncbi.nlm.nih.gov)
Cannon also distinguished homeostasis from simple physical equilibrium. Organisms are open systems, exchanging matter and energy with their surroundings. Their relatively stable internal conditions depend on continuing activity by multiple cooperating mechanisms. Consequently, homeostasis should not be equated with thermodynamic equilibrium, in which there is no net driving force for further macroscopic change. (pmc.ncbi.nlm.nih.gov)
Regulatory organization
A basic homeostatic mechanism can be described using three functional components: a sensor, an integrating or control process, and an effector. The sensor detects a regulated variable or a signal associated with it. The integrating process determines the response, and the effector changes physiological activity. These functions need not occupy separate organs; a single cell may both detect a disturbance and initiate a corrective response. (openstax.org)
Most familiar examples involve negative feedback. A rise in a regulated variable elicits responses tending to lower it, or a fall elicits responses tending to raise it. “Negative” describes the opposing direction of the response, not an undesirable effect. As the disturbance diminishes, the corrective response generally weakens. A set point is a useful model for the value around which regulation occurs, although a physiological system may maintain a range rather than one exact value. (openstax.org)
Positive feedback instead amplifies an initiating change. During childbirth, cervical stretching promotes oxytocin release, which strengthens contractions and produces further stretching. Delivery removes the initiating stimulus. Such processes help complete particular events, but amplification alone does not provide the stabilizing action characteristic of negative-feedback regulation. (openstax.org)
Major physiological examples
Temperature regulation
In humans and other endothermic animals, thermoregulation balances heat production with heat exchange. Temperature-sensitive structures provide information to coordinating centers, including the hypothalamus. Sweating and increased skin blood flow promote heat loss, whereas reduced skin blood flow and shivering help conserve or generate heat. Behavioral responses, such as seeking shade or shelter, also contribute. These mechanisms regulate internal temperature without making it completely independent of environmental conditions. (openstax.org)
Blood glucose regulation
The pancreas participates in regulating circulating glucose. When glucose rises, pancreatic beta cells increase insulin secretion. Insulin promotes glucose uptake in insulin-responsive tissues and favors storage, helping reduce circulating glucose. When glucose falls, pancreatic alpha cells release glucagon, which promotes glucose production and release by the liver. The opposing actions of these hormones connect nutrient availability with the requirements of metabolism. (openstax.org)
Water balance
The regulation of water involves both intake and excretion. Osmoreceptors detect changes in the concentration of body fluids, contributing to thirst and the release of antidiuretic hormone, also called vasopressin. This hormone increases water reabsorption by the kidneys, partly by controlling the insertion of water-channel proteins into collecting-duct cell membranes. Increased retention reduces water loss in urine; reduced hormone activity permits greater water excretion. (openstax.org)
Acid–base regulation
Blood pH is maintained through interacting chemical and physiological mechanisms. Chemical buffers limit abrupt changes in acidity. The lungs regulate the removal of carbon dioxide, which participates in the carbonic acid–bicarbonate system. The kidneys regulate hydrogen-ion excretion and bicarbonate handling. These mechanisms operate on different timescales, illustrating how several processes can jointly stabilize one internal variable. (openstax.org)
Dynamic stability and limits
Homeostatic ranges are not necessarily fixed throughout life or across circumstances. Biological signaling can modify regulatory activity and the capacity to tolerate disturbances. Research on adaptive homeostasis examines temporary expansion or contraction of this capacity in response to non-damaging signals. Such adjustments preserve functioning through change rather than requiring every variable to remain at its previous value. (pmc.ncbi.nlm.nih.gov)
Regulation also has finite capacity. A disturbance may exceed the available response, or a component of the regulatory system may function inadequately. In acid–base regulation, for example, impaired kidney or respiratory function can limit compensation. Homeostatic disruption therefore refers to failure to maintain an appropriate internal range, not simply to any departure from an average measurement. (openstax.org)
Scope of the concept
In its original organismal sense, homeostasis concerns variables of the internal environment and the mechanisms maintaining them. Physiological usage also extends to cellular regulation. Identifying the system level and the particular regulated variable is important: merely observing a stable condition does not establish which processes maintain it. A mechanistic account specifies the disturbance, the sensing and response pathways, and how their actions oppose or accommodate that disturbance. (journals.physiology.org)