homeostasis

The human body maintains core temperature within a range of less than 1°C (1.8°F) despite external temperatures swinging 60°C or more across seasons, and despite internal heat generation that — in hard exercise — rises to 20 times the resting rate. This precision requires a thermostat, an active cooling system, an emergency heating system, and dedicated fat that burns itself to generate warmth. None of it is conscious. Here is how thermoregulation actually works.

Your kidneys filter your entire blood supply roughly 40 times every day — about 180 liters of fluid — and return 99% of it to the bloodstream while excreting only 1 to 2 liters as urine. The precision of that selective reclamation process is what keeps you alive.