Homeostasis

Homeostasis (also spelled homoeostasis) is the biological term for the stable internal physical and chemical conditions that living organisms maintain. This state allows an organism to function optimally, with variables like body temperature and fluid balance held within specific pre-set limits—known as the homeostatic range.

Sodium concentration

Other regulated variables include the pH of extracellular fluid, the levels of sodium, potassium, and calcium ions, and blood sugar concentration. These must be controlled despite changes in the environment, diet, or activity level. Each variable is managed by one or more homeostatic mechanisms, which together sustain life.

Overview

Homeostasis works through a natural resistance to change when conditions are already optimal, and equilibrium is upheld by many regulatory processes. It is considered the central motivation for all organic action. Every homeostatic control mechanism has at least three interdependent components for the variable being regulated: a receptor, a control center, and an effector. The receptor senses and responds to changes in the internal or external environment—examples include thermoreceptors and mechanoreceptors.

Control centers include the respiratory center and the renin-angiotensin system. The effector is the target that acts to return the variable to normal; at the cellular level, effectors include nuclear receptors that adjust gene expression via up-regulation or down-regulation, operating in negative feedback loops. An example is the control of bile acids in the liver.

Some control centers, like the renin-angiotensin system, regulate more than one variable. When a receptor detects a stimulus, it sends action potentials to a control center. The control center sets the acceptable upper and lower limits for that variable—such as temperature—and then determines an appropriate response, sending signals to an effector (which may be muscles, an organ, or a gland). Once the effector acts, negative feedback informs the receptor to stop further signaling.

The cannabinoid receptor type 1, found on presynaptic neurons, helps stop stressful neurotransmitter release to the postsynaptic neuron. It is activated by endocannabinoids like anandamide and 2-arachidonoylglycerol through retrograde signaling: these compounds are made and released by postsynaptic neurons, travel back to the presynaptic terminal, and bind to the CB1 receptor to fine-tune neurotransmitter release and achieve homeostasis. Polyunsaturated fatty acids—omega-3 derivatives (docosahexaenoic acid and eicosapentaenoic acid) and omega-6 derivatives (arachidonic acid)—are synthesized from membrane phospholipids and serve as precursors for endocannabinoids, playing a key role in the body’s homeostatic adjustments.

History

The word homeostasis comes from Neo-Latin, combining Greek homoios ("similar") and stasis ("standing still"), meaning "staying the same." The concept of regulating the internal environment was first described by French physiologist Claude Bernard in 1849, and the term homeostasis was coined by Walter Bradford Cannon in 1926. In 1932, British physiologist Joseph Barcroft was the first to state that higher brain function requires the most stable internal environment; for him, homeostasis was not only organized by the brain but also served the brain. Homeostasis is almost exclusively a biological term, referring to Bernard and Cannon’s ideas about the constancy of the internal environment in which cells live. The term cybernetics applies to technological control systems like thermostats, which function similarly but are defined more broadly.

Metabolic processes in all organisms occur only within very specific physical and chemical conditions. These conditions vary by organism and by whether processes happen inside cells or in the interstitial fluid around them.

In humans and other mammals, the best-known homeostatic mechanisms keep the extracellular fluid (the "internal environment") constant, especially regarding temperature, pH, osmolality, and concentrations of sodium, potassium, glucose, carbon dioxide, and oxygen. Many other homeostatic mechanisms control additional aspects of physiology. When variable levels are too high or too low, they are often prefixed with hyper- or hypo-, such as hyperthermia/hypothermia or hypertension/hypotension.

Being homeostatically controlled does not mean a value is absolutely steady in health. Core body temperature, for example, is regulated by a homeostatic mechanism with temperature sensors in the hypothalamus and elsewhere. However, the set point is regularly reset: human core body temperature varies throughout the day (a circadian rhythm), with lowest temperatures at night and highest in the afternoons. Other normal variations include those linked to the menstrual cycle.