Homeostasis

Homeostasis is the maintenance of a stable internal environment despite changing external conditions. Body temperature, blood glucose, blood pH, fluid balance, electrolytes and blood pressure are all held within narrow ranges by systems that continuously sense and correct.

Almost all of it works by negative feedback. A sensor detects a deviation, a control centre compares it with a set point, and effectors act to reverse it, at which point the correction winds down. Sweating when hot and shivering when cold are the schoolroom example, and insulin and glucagon holding blood glucose steady are the version that matters most on this site.

It explains why so many single interventions produce less than expected. Push on one variable and the system pushes back, which is why eating less reduces spontaneous movement, why weight loss lowers energy expenditure beyond what size predicts, and why the body defends its previous state rather than accepting a new one.

It also explains why some numbers cannot be moved by diet at all. Blood pH is defended within a very narrow range by respiratory and renal mechanisms, which is why alkaline diets cannot change it, whatever they claim.

Training works with it rather than against it. Exercise is a controlled disturbance, and the adaptation that follows is the system rebuilding itself to handle that disturbance more easily next time, which is the entire logic of progressive overload.

Set points are not entirely fixed. They shift with age, with adaptation, and in disease, and part of what long term training does is move them.

In practice, expect resistance whenever you change something substantially, and plan for it rather than treating it as failure.