The skin contains a network of small blood vessels that have tiny muscles inside the walls. These muscles are under the control of the sympathetic nervous system. Blood flow to the skin provides nutrition to the skin and regulates body heat. The blood vessels in the skin act as the body's heating and cooling system by controlling the amount of blood flow to the skin Usually, the muscles are partly contracted (squeezed). If the muscles contract more than normal, for example in cold weather, the blood vessels also contract (vasoconstriction) so that less blood passes through them, reducing the loss of body heat. If the blood flow is restricted, the skin becomes pale and white. When the muscles are completely relaxed, for example in warm weather, the blood vessels dilate or widen (vasodilation). This allows more blood to pass through the skin resulting in the body radiating more heat thus making the skin appear red.
The circulatory system of the skin contains three major types of blood vessels: arteries, capillaries and veins, which serve mainly nutrition needs. The subcutaneous venous plexus plays a major role in the conduction of heat and contains a major fraction of the cutaneous blood volume. Arteriovenous anastomoses can be found in areas of the body exposed to maximal cooling, such as the hands, feet, nose, lips and ears. These areas are called apical structures and are richly innervated. The anastomoses connect cutaneous arterioles and venules directly, playing an important role in the reduction of blood flow in a cold environment.