The body defends a number
Your blood sugar, your blood calcium, your body temperature, the amount of water in your blood — every one of these sits near a target value and is held there with surprising tightness. That target is the set point, and the act of keeping a variable near it is homeostasis. Think of a thermostat set to 21°C: it does not care what the weather does, it just keeps acting until the room reads 21 again.
Hormones are the body's main long-distance way of doing this. A hormone is a chemical message poured into the blood; when a regulated variable drifts off its set point, the relevant gland changes how much it releases, and the variable is nudged back. The control system has three parts you will meet again and again: a sensor that measures the variable, a comparator that knows the set point, and an effector that does something about the gap.
Why control needs two directions
A single hormone that can only push one way is a poor regulator. To hold a value steady you need to correct overshoots and undershoots. That is why most regulated variables are guarded by a pair of opposing hormones, or by a single hormone whose secretion can rise far above and fall far below a resting level.
- Blood glucose is held by insulin (lowers it) and glucagon (raises it) — opposing hormones pulling the same number in two directions.
- Blood calcium is held by PTH (raises it) and, more modestly, calcitonin (lowers it), with vitamin D as a slower partner.
- Body water is held mainly by vasopressin (ADH), which can be turned strongly up to conserve water and switched almost off to dump it.
Variable drifts HIGH Variable drifts LOW | | v v sensor detects rise sensor detects fall | | v v gland that LOWERS it turns ON gland that RAISES it turns ON (e.g. insulin for glucose) (e.g. glucagon for glucose) | | v v variable falls back toward variable rises back toward SET POINT <-----------------------> SET POINT