= Homeostasis
{wiki}
<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. <Anatomy and Physiology 2e by J. Gordon Betts and others/1.5 Homeostasis>[§1.5, opening paragraphs and Negative Feedback: normal ranges and compensatory responses]{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. <Anatomy and Physiology 2e by J. Gordon Betts and others/1.5 Homeostasis>[§1.5, Negative Feedback, Figure 1.10: sensor, control center and effector]{ref}
\Image[/homeostasis-temperature-feedback-circuit.png]
{title=<Homeostasis>: sensing, integration and action, with a synthetic disturbance response}
{description=Original qualitative <thermoregulation> circuit above an explicitly synthetic plot from the <homeostatic temperature model>. The solid return represents <negative feedback> from <core body temperature>; the dashed <skin> input can anticipate a core change. The lower curves use invented <dimensionless variables> and arbitrary time units. They are neither experimental measurements nor fitted human responses.}
<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 https://www.panarchy.org/cannon/homeostasis.1932.html[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. <The Wisdom of the Body by Walter Bradford Cannon/Homeostasis (excerpt from The Wisdom of the Body)>[Homeostasis excerpt, paragraphs beginning The wonder increases, Resistance to changes and In short; hosting page identifies book and 1932 date]{ref}
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