Changes in Capillary Blood Flow for Heat Loss

Body temperature is monitored and controlled by the thermoregulatory centre in the brain. This center has receptors sensitive to the temperature of the blood flowing through the brain. If the core body temperature is too high, the thermoregulatory center causes the blood vessels supplying the skin capillaries to expand so that more blood flows through them. Since the blood flows through the entire body, it can take the excess heat from the core of the body to the skin, where it can be lost. When a blood vessel of any sort increases in diameter, we say that it dilates. Because the dilation is related to the vascular (blood) system, this process is called vasodilation.

Vasodilation means that more blood will flow through the capillaries in the surface layers of the skin, which is why most people look redder during exercise. The extra volume of blood heats up the surface of the skin more, so more heat is lost from the body by radiation and convection. During exercise, the sweat glands release more sweat onto the surface of the skin to cool the body down. Because the capillaries are carrying more blood, heating the surface of the skin, the sweat evaporates faster which cools the body more effectively.

If the core temperature of the body drops, exactly the opposite happens (vasoconstriction) in order to try to prevent hypothermia. This process prevents the blood from carrying precious body heat to the surface of the skin where it would be lost. At high altitudes, mountaineers have much lower levels of oxygen in their blood than normal which means that they can only move slowly. This, together with the low temperatures found on mountains, makes it difficult for them to maintain their body temperature and so vasoconstriction occurs. It is a big problem for their toes and fingers as these become intensely cold. The blood flow virtually disappears and if it continues for a long period of time, without the vital oxygen, the cells begin to die. This is called frostbite and the dead tissue turns black and either falls off naturally or is removed surgically. Many mountaineers have lost fingers and toes to frostbite.

How do changes in the size of capillaries help with heat loss and heat retention? Which part of the brain controls our body temperature? Find out if you know by playing this quiz.