28.21 : Thermoregulation

The human body has a sophisticated thermoregulation system that employs negative feedback mechanisms to maintain an optimal core temperature. When the core temperature drops, peripheral and central thermoreceptors send signals to the hypothalamus, activating the heat-promoting center. This center triggers several responses aimed at increasing the core temperature. First, vasoconstriction reduces the flow of warm blood from internal organs to the skin so that the heat is not lost from the skin, thereby conserving heat. The hypothalamus also triggers the release of hormones epinephrine and norepinephrine, which boost cellular metabolism and, consequently, heat production. Shivering is another mechanism where the brain increases muscle tone, producing heat through a cycle of small contractions and stretch reflexes. Lastly, the hypothalamus releases thyroid-stimulating hormone (TSH), causing the thyroid gland to release more thyroid hormones into the blood, slowly increasing the metabolic rate and body temperature.

On the other hand, when the core temperature rises above normal, a different set of negative feedback mechanisms are activated. Peripheral and central thermoreceptors relay signals to the hypothalamus, stimulating the heat-losing center and inhibiting the heat-promoting center. As a result, there is vasodilation. There is an increase in the blood flow to the skin, and excess heat is lost to the environment. Metabolic rate decreases, and shivering ceases. Additionally, the hypothalamus activates sweat glands, and as the water in perspiration evaporates, the skin cools down, helping return body temperature to normal.