The Circulatory System and Thermal Homeostasis

While the circulatory system transports (de)oxygenated blood around the body, it also transports heat. The flow of blood through vessels placed near the skin’s surface allows some heat loss to the external environment, through a variety of heat exchange methods.

Ever wondered why you turn red when you exercise? Heavy exercise generates heat which is often lost by “raising” the blood vessels near the skin, this process is known as vasodilation. Vasodilation of surface-area blood vessels increases the volume of blood passing a single point at the skin’s surface and so maximizes the amount of blood that contacts the external environment (air or water). Increased blood flow near the surface of the skin accelerates the potential for heat exchange with the environment. Vasodilation causes increased heat loss at the extremities of the body, eg. the ears and toes, which have high surface area to volume ratios. The rate of heat loss across the skin can also be accelerated by increasing the rate of fluid flow across the outside of the skin, for example by high winds. This is why it feels colder when the wind increases in intensity, even when the outside temperature has not changed.

In contrast to vasodilation, vasoconstriction of surface area blood vessels reduces the amount of warm blood reaching the body surface and therefore, reduces the amount of heat exchange with the environment.

In addition to vasodilation and vasoconstriction, specialized blood vessels, called countercurrent exchangers also help regulate convective and radiative heat loss and especially help reduce the amount of heat lost through contact with ice and cold ground.