Thermoregulation

Thermoregulation is what the body does so that its core temperature, the temperature inside, stays almost the same even when everything outside changes. The numbers give an idea of the margin: a naked person can stand in dry air at 12.8 °C or at 54.4 °C and the core barely moves, because every temperature-control mechanism works to bring it back to its set point.

To do that, the body has to measure. In the anterior preoptic hypothalamic area, a region of the hypothalamus, which is a small zone at the base of the brain, there are neurons sensitive to cold and others sensitive to heat. Those signals travel to the posterior hypothalamus, which combines them with the ones arriving from the periphery, that is from receptors spread through the skin and the deep tissues.

From there come the orders, and they come in two opposite sets. If core temperature rises, the skin vessels dilate and sweating increases. If it falls, those same vessels constrict and shivering and chemical thermogenesis appear, the latter being heat produced by speeding up the body's reactions. The skin is the radiator; blood flow to it is the valve.