thermoregulation

Meaning: Thermoregulation is the maintenance of a fairly stable core body temperature despite a changing environment. It is homeostasis by negative feedback. Thermoreceptors in the skin and hypothalamus send input to the hypothalamus, which compares temperature with a set point and signals effectors. If the body is too warm, skin arterioles dilate, sweat glands secrete, and evaporation carries heat away. If too cold, arterioles constrict, skeletal muscle shivers, and brown fat can release heat (nonshivering thermogenesis). Endotherms produce much of their heat metabolically; ectotherms rely largely on behavior such as basking or seeking shade.

On the MCAT: Expect a passage about exercise, cold exposure, or a hypothalamic change, then a question on which response follows, such as the direction of skin blood flow. Sweating cools because evaporation removes heat, so high humidity reduces its effect. Countercurrent exchange in limbs, where arteries transfer heat to returning venous blood, also appears as a heat-conserving design.

Common confusion: Students think vasoconstriction cools the body. In heat, vasodilation brings warm blood near the skin surface so more heat is lost; vasoconstriction in cold reduces skin blood flow and conserves heat. Also, ectotherms ("cold-blooded") do regulate temperature, mainly through behavior.