The constancy of the internal environment is the condition for a free and independent life. ~ Claude Bernard
In the mid-19th century, French physiologist Claude Bernard aimed to establish the rigor of the scientific method in medicine. In doing so, he dismissed many previous misconceptions via experimental results.
Contrary to prevailing thought at the time, Bernard was convinced that all living creatures were bound by the same physical laws as inanimate matter.
Bernard conceived Milieu intérieur: a body’s self-regulation of its internal environment allowed considerable independence from the external environment.
In his 1932 book The Wisdom of the Body, American physiologist Walter B. Cannon captured the concept in a word, coining the term homeostasis.
Most animals constantly lose fluid by evaporation, urination, and other means. In a feedback loop, that loss is registered by receptors, and the body initiates action to compensate by drinking water or other fluid intake.
Homeostatic models assume a static ideal set point. However helpful in facilitating conceptual understanding, such models are necessarily gross simplifications of real life.
Model behavior would indicate that dehydrated animals would drink until rehydration happens, then stop. But something subtler and complex happens.
A human deprived of water for 24 hours will drink enough to restore fluid balance within 2.5 minutes, and then stop drinking, even though changes in plasma dilution cannot be physiologically detected for 7.5 minutes, and fluid balance is not back to normal until about 12.5 minutes. There is no purely physiological explanation for this.
Homeostatic regulation applies to innumerable facets of living: from temperature to fluid to chemicals to light. All bodily functions involve intricate sets of trade-offs in ongoing homeostatic actions, including host cell nutrition and replacement that coincide with the homeostatic needs of the microbiome within.
Many maladies are a cascade effect from homeostasis disturbed: homeostatic imbalance. Organisms lose efficiency in their control systems as they age. Such inefficiencies accumulate into a less stable internal environment that increases the risk of illness. Aging is an output of homeostatic imbalance.