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Last updated: Sat, Mar 15, 2025
The autonomic nervous system has both homeostatic and allostatic functions.
Homeostasis is the maintenance of metabolic parameters (such as ion concentrations, blood glucose, blood gases, blood pressure, body temperature, and digestive processes) within a narrow range. (Your furnace thermostat performs a homeostatic function by keeping the house temperature at the level you specify.) Homeostatic regulation is managed largely by the brain stem with higher input from many of the same brain areas used in the pain network. The autonomic system controls the metabolic parameters just listed as well as others. The system has several means by which it performs regulation.
The autonomic system includes direct neural connections to the heart, the bronchial tubes, stomach, kidneys, intestines and bladder. With these direct connections it can change the heart rate as well as the strength of heart contractions; enlarge or constrict the bronchial tubes at the entry to the lungs; increase or decrease kidney output; and speed or retard the flow of foodstuffs through the digestive system.
It also has neural connections to most of the blood vessels of the body, to the involuntary muscles that are attached to most hairs, and to the sweat glands. It can restrict or dilate the blood vessels to raise or lower blood pressure, to control which areas of the body get more or less blood flow, and to regulate loss of heat to the environment. It can make all of your hairs stand on end (this is called piloerection) or turn your sweat glands on and off, also for heat regulation.
Allostasis is the adjustment of metabolic and other processes to cope with stresses and other challenges, such as exercise, hunger, extreme temperatures, physical threat, or pain. Allostatic adaptation is turned on when needed and turned off when not. If allostatic mechanisms are used for long periods, it is called allostatic load. Overuse of allostatic mechanisms leads to diseases such as hypertension, obesity, or diabetes.
The autonomic system also has neural connections to the adrenal glands. It can use these to cause the adrenal glands to excrete small or large amounts of epinephrine (adrenalin) and norepinephrine (a relative of epinephrine) into the blood stream.