Homeostasis is the maintenance of stable internal conditions in a living organism despite changes in the external environment. Body temperature stays near 37°C whether the air is 0°C or 40°C. Blood pH stays between 7.35 and 7.45 regardless of what you eat. Blood glucose stays within a narrow range between meals. These are not passive equilibria — they are actively maintained by physiological control systems that detect deviations and correct them.
The homeostatic loop
Every homeostatic mechanism follows the same basic structure:
- Sensor — detects the current value of the variable being regulated (thermoreceptors detect body temperature; chemoreceptors detect blood CO2; baroreceptors detect blood pressure)
- Control center — compares the detected value to a set point and determines the appropriate response (the hypothalamus for temperature; brainstem respiratory centers for CO2; cardiovascular control centers for blood pressure)
- Effector — carries out the corrective response (sweat glands and blood vessel dilation for cooling; increased respiratory rate for CO2 elimination; heart rate and vessel tone adjustment for blood pressure)
Most homeostatic mechanisms use negative feedback — the response opposes the deviation, pushing the variable back toward the set point. Rising body temperature triggers sweating and vasodilation, which lower temperature. Falling blood glucose triggers glucagon release, which raises glucose. The response is always in the opposite direction of the change.
Homeostasis and the autonomic nervous system
The autonomic nervous system is the primary executor of homeostatic regulation. Its two divisions produce opposing effects:
- Sympathetic nervous system — prepares the body for action (“fight or flight”): increases heart rate, dilates bronchi, diverts blood flow to muscles, releases glucose from liver stores, dilates pupils
- Parasympathetic nervous system — promotes maintenance and recovery (“rest and digest”): decreases heart rate, constricts bronchi, promotes digestion, constricts pupils
These divisions do not alternate between on and off. Both are active continuously, with their balance shifting based on context. Somatic Experiencing works directly with autonomic regulation — helping the nervous system complete incomplete defensive responses and restore the autonomic flexibility that trauma can disrupt.
Homeostasis, allostasis, and disease
Allostasis extends the homeostasis concept: rather than maintaining a fixed set point, the body adjusts its set points in response to anticipated demands. The stress response raises blood pressure, mobilizes glucose, and suppresses non-essential functions (digestion, reproduction, immune activity) in anticipation of a threat. This is adaptive in the short term — the body prepares for action.
Allostatic load is the cumulative cost of repeated allostatic adjustments — the wear on the body when stress responses are activated chronically. Sustained cortisol elevation suppresses immune function, promotes visceral fat deposition, impairs memory, and — critically for this module — shifts descending pain modulation toward facilitation, increasing vulnerability to chronic pain. Allostatic load is the physiological mechanism through which poverty, discrimination, and structural abandonment produce disease.
Homeostasis and TCM
In traditional Chinese medicine, the concept closest to homeostasis is the dynamic balance of Yin and Yang — complementary aspects of any phenomenon whose relative balance determines health. Disease results from their imbalance: excess Yang produces heat, agitation, and inflammation; excess Yin produces cold, stagnation, and depletion. Treatment aims to restore balance — not to a fixed set point but to a dynamic equilibrium appropriate to the patient’s constitution and circumstances.
The Three Treasures framework extends this: Jing (stored substrate) must be adequate, Qi (operational activity) must flow, and Shen (reflective awareness) must be present. A system that has substance and activity but lacks the capacity to assess whether its own operations still serve the situation is a system whose homeostasis has become maladaptive — maintaining a set point that no longer fits.
Related terms
- Organ — the organ systems that maintain homeostasis
- Inflammation — a homeostatic disruption that is itself a protective mechanism
- Cell — the units whose internal conditions homeostasis maintains