Vital Signals: Temperature
Man, other mammals and birds are "warm-blooded" (homeothermic—unchanging temperature), meaning that although exposed to a variety of temperatures the body remains at a certain warm temperature. Cells of the body function optimally within a narrow range (97.7-100.4˚F/ 36.5-38˚C), and mammals and birds have evolved a way of regulating the internal temperature to stay within this span.
In contrast, amphibians, reptiles and fish ("cold-blooded" or poikilothermic—variable temperature) possess less sophisticated regulatory systems, take on the temperature of the
environment and must take radical moves to avoid extremes of hot and cold (actually "cold-blooded" is a misnomer, since frogs, snakes and fish have warm blood).
The temperature regulatory center in man is the hypothalamus, an area of the brain above the pituitary gland which acts as a thermostat to maintain the body’s internal, or core, temperature at a range of about 97-99.8˚F/ 36.1-37.7˚C.