User:Anthony.Sebastian/Homeostasis/Sebastian Notes/Archive 1

Referring to animal systems, pioneering physiologist Walter Cannon, who coined the word homeostasis in 1926, defined it as follows:

The coordinated physiological reactions which maintain most of the steady states in the body are so complex, and are so peculiar to the living organism, that it has been suggested (Cannon, 1926) that a specific designation for these states be employed--homeostasis.

Cannon recognized that “living being[s]” function as ‘open’ systems (see Life) with many “relations” with their surroundings — for example through airways, gastrointestinal tract and skin. He noted that the surroundings could perturb the system to move its key components or subsystems to points outside of their relatively stable and optimal ranges of property or function — the “steady states” in his definition. A change in outside temperature, for example, might perturb the stability of admittedly dynamical biochemical processes to the detriment of the organism. The organism reacts to such potentially adverse effects of it surroundings with physiological adjustments that tend to maintain steady-state, i.e., to maintain 'homeostasis'.

This article will explore the concept of homeostasis in an early 21st century biological setting, exemplify ‘homeostatic’ (homeostasis-maintaining) mechanisms, and relate homeostasis to the concepts of cellular and organismic adaptation, growth, development and reproduction.

Terminology and examples of usage

In common usage, ‘homeostasis’ refers not only to a living system’s stability, but also to its ability to, or tendency to, maintain stability. Because living systems can adjust their physiological processes in response to destabilizing perturbations, we can attribute to them the property or behavior of ‘homeostasis’, by which property or behavior it maintains ‘homeostasis’. Such characterizes the vagaries, or flexibility, of language.

Use of the adjectival form, homeostatic, adds a modicum of coherence. Typically biologists speak of ‘homeostatic mechanisms’, namely mechanisms that achieve, or tend to achieve, internal stability. They do not typically speak of homeostatic mechanisms as mechanisms that yield an organism’s ‘ability’ to make the physiological adjustments required for stability. To explain that requires investigation of principles that underpin the activity of living itself, including evolutionary forces enabling self-organization and autonomy (see Life).

In its narrow sense applied to living systems, homeostatic mechanisms operate as built-in autonomous physiological processes, goal-directed to maintain, within an optimal range or steady-state, the properties, functions or behaviors of the system’s key components or subsystems when perturbed to move outside that ‘set-point’ range, or ‘set-range’.

For a cell, homeostatic mechanisms operate, for example, to maintain its internal concentration of hydrogen ion near its optimal value (set-point), the failure of which would have widespread effects on the naturally-selected catalytic activity of enzymes necessary for cell organization and survival. For a multicellular organism, homeostatic mechanisms operate, for example, to maintain near optimal supplies of oxygen to an organ, the failure of which could lead to organ dysfunction and cascading deleterious effects on other organs.

In its broader sense applied to living systems, homeostatic mechanisms operate as the totality of those physiological adjustive processes that ensure the near steady-state functioning required to sustain the activity of living. Because living systems grow in time, behave differently in different environments, and manufacture living things like themselves, set-point optimality does not remain fixed. Homeostatic mechanisms can accommodate the changes, indicating their complexity.