Thermoregulation by the skin

Thermoreceptors in the skin (free nerve endings) and in the core (hypothalamus, spinal cord, viscera) feed the preoptic area of the anterior hypothalamus, which compares body temperature with a set point (≈37 °C) and drives effectors through the sympathetic nervous system.

Heat loss. At rest in a comfortable environment ≈60% of heat is lost by radiation, ≈15% by conduction/convection and ≈20–25% by evaporation (insensible loss from skin and airways plus sweat). When too hot, sympathetic vasoconstrictor tone is withdrawn and the active vasodilator system opens skin vessels, bringing warm core blood to the surface; eccrine sweating (sympathetic cholinergic) wets the skin, and each gram evaporated removes ≈2.4 kJ. When ambient temperature exceeds skin temperature, evaporation becomes the only effective route, so high humidity is dangerous. Heat acclimatisation over ≈1–2 weeks increases sweat rate, starts sweating at a lower temperature and lowers sweat NaCl (aldosterone).

Heat conservation. When cold, noradrenergic vasoconstriction of dermal arterioles and AV anastomoses shrinks skin blood flow and the countercurrent arrangement of deep limb veins and arteries returns heat to the core; piloerection (arrector pili) is vestigial in humans; subcutaneous fat insulates. Heat production increases by shivering (skeletal muscle) and non-shivering thermogenesis (brown fat, thyroid hormones) — effectors outside the skin.