The primary function of the skin circulation is to help maintain body temperature. Blood vessels constrict to prevent heat loss and dilate to facilitate transfer of heat from the body core to the body surface. The skin comprises 4% to 5% of the total body weight and receives about 2% of the cardiac output. The arterio-venous oxygen difference is small (3 Vol%), indicating that most of the blood flow is nonnutrient flow.
Skin blood vessels (arterioles) are of two types. The most numerous are composed of smooth muscle, supply capillary beds, and are innervated with sympathetic constrictor fibers, like arterioles in skeletal muscle beds. They provide nutrient flow to the skin. Veins draining these vascular beds comprise large venous plexuses with slow blood flow in the forearms, legs, and thighs. These plexuses provide a large surface area for heat exchange with the environment. The second type of vessel is composed almost exclusively of smooth muscle. These provide a direct connection between arteries and the venous plexuses described above. They are known as arterio-venous anastamosies (AVA's) and are numerous in the palms, soles, and skin of the ears, nose, and lips. Flow is nonnutrient because there are few exchange vessels. These vessels have a low level of basal tone and are innervated exclusively by sympathetic fibers, to which they are very responsive.