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Homeostasis is the maintenance of usable internal conditions despite disturbances. It concerns selected regulated physiological variables, rather than keeping every part of an organism unchanged: core body temperature may remain near its usual range while skin temperature, sweating and blood pressure change. Regulation makes a relatively dependable environment for biological cells out of a changing external world. ^[ref]

A useful causal description separates a physiological sensor, a physiological integrator and a physiological effector. The return from the physiological effector's action to the sensed regulated physiological variable closes a feedback control loop. In negative feedback, that return opposes the disturbance. The insulin and glucagon articles already trace the blood glucose example; synaptic integration explains how several inputs can jointly influence a neuron. These are implementations and ingredients, not a requirement that every regulatory loop use one anatomical control center. ^[ref]

Homeostasis is useful precisely because internal activity can change: a nearly steady temperature can conceal a large increase in heat transfer. Conversely, observing a flat trace alone does not identify a regulatory mechanism. The environment might simply be constant, or the measuring instrument too insensitive; a perturbation and an independent response readout provide stronger evidence. The skin-temperature perturbation experiment below makes that distinction tangible.

The historical motivation was the puzzle of stability in changing living material. In his 1932 The Wisdom of the Body, Walter Bradford Cannon emphasized that an organism exchanges material with its surroundings and continually replaces its own structure, yet maintains relatively steady internal conditions. This authored excerpt presents homeostasis as active compensation for external and internal disturbances, rather than a static body. ^[ref]

A physiological effector performs the action that changes the regulated physiological variable. In thermoregulation, sweat glands, changes in skin blood flow, and skeletal muscle shivering contribute different actions. The command, the action and the resulting temperature are three distinct readouts. ^[ref]

A physiological integrator combines regulatory inputs to select a response; it need not perform the mathematical integral action used in control theory. In thermoregulation, the hypothalamus participates in combining internal and skin signals. The term describes a role, which can be distributed across interacting biological cells, rather than a mandatory single comparator.

A physiological sensor detects a relevant condition and changes a signal. In thermoregulation, temperature-sensitive neurons supply information about internal and skin temperature. Sensing a disturbance and sensing its internal consequence need not be the same operation.

A regulated physiological variable is a specified internal quantity whose departures elicit actions tending to restore an appropriate range. Examples include core body temperature, blood glucose concentration, blood pressure, and the pH and osmolality of extracellular fluid. Specify the compartment and readout: skin temperature and core body temperature are different quantities, and an insulin concentration is a regulatory message rather than the blood glucose it helps control.

Thermoregulation regulates body temperature. Skin signals can anticipate a change of core body temperature, so a response need not wait for an internal error. ^[ref]

These responses have costs and environmental limits. Shivering consumes metabolic resources; cooling through sweating removes water that must eventually be replaced. High humidity restricts evaporation, so secreted sweat and successful cooling are not interchangeable. Maintaining core body temperature also depends on maintaining circulation: depleted blood volume can compromise skin blood flow and heat transfer. Seeking shade or reducing work changes the disturbance itself, whereas a larger regulatory command merely attempts to counter it. ^[ref]