allostasis

Allostasis means staying stable by changing. Where the older idea of homeostasis pictures the body defending fixed set points, allostasis emphasizes that the body actively shifts its internal settings to suit the situation: blood pressure should be higher when you sprint and lower when you sleep. Stability, in this view, is achieved not by holding still but by anticipating demand and adjusting in advance.

Stress hormones are central tools of allostasis. Cortisol and catecholamines do not push the body back toward a single resting point; instead they re-tune metabolism, cardiovascular function, and the immune system to whatever the brain predicts is coming. The brain itself is the chief allostatic organ, because it forecasts demand and orchestrates the hormonal and autonomic adjustments.

The term was introduced by Sterling and Eyer in 1988 and developed by Bruce McEwen. Allostasis is best seen not as a replacement for homeostasis but as a complementary lens: some variables are defended around fixed points, while others are deliberately moved to keep the whole organism functioning.