The skin is a large organ, representing 10 to 15% of total body mass. The primary functions of the skin are protection of the body from the external environment and dissipation or conservation of heat during body temperature regulation.
The skin has one of the lowest metabolic rates in the body and requires relatively little blood flow for purely nutritive functions. Consequently, despite its large mass, its resting metabolism does not place a major flow demand on the cardiovascular system. However, in warm climates, body temperature regulation requires that warm blood from the body core be carried to the external surface, where heat transfer to the environment can occur. Therefore, at typical indoor temperatures and during warm weather, skin blood flow is usually far in excess of the need for tissue nutrition. The reddish color of the skin during exercise in a warm environment reflects the large blood flow and dilation of skin arterioles and venules (see Table 17.1).
The increase in the skin's blood flow probably occurs through two main mechanisms. First, an increase in body core temperature causes a reflex increase in the activity of sympathetic cholinergic nerves, which release acetyl-choline. Acetylcholine release near sweat glands leads to the breakdown of a plasma protein (kininogen) to form bradykinin, a potent dilator of skin blood vessels, which increases the release of NO as a major component of the dilatory mechanism. Second, simply increasing skin temperature will cause the blood vessels to dilate. This can result from heat applied to the skin from the external environment, heat from underlying active skeletal muscle, or increased blood temperature as it enters the skin.
Total skin blood flows of 5 to 8 L/min have been estimated in humans during vigorous exercise in a hot environment. During mild to moderate exercise in a warm environment, skin blood flow can equal or exceed blood flow to the skeletal muscles. Exercise tolerance can, therefore, be lower in a warm environment because the vascular resistance of the skin and muscle is too low to maintain an appropriate arterial blood pressure, even at maximum cardiac output. One of the adaptations to exercise is an ability to increase blood flow in skin and dissipate more heat. In addition, aerobically trained humans are capable of higher sweat production rates, this increases heat loss and induces greater vasodilation of the skin arterioles.
The vast majority of humans live in cool to cold regions, where body heat conservation is imperative. The sensation of cool or cold skin, or a lowered body core temperature, elicits a reflex increase in sympathetic nerve activity, which causes vasoconstriction of blood vessels in the skin. Heat loss is minimized because the skin becomes a poorly perfused insulator, rather than a heat dissipator. As long as the skin temperature is higher than about 10 to 13°C (50 to 55°F), the neurally induced vasoconstriction is sustained. However, at lower tissue temperatures, the vascular smooth muscle cells progressively lose their contractile ability, and the vessels passively dilate to various extents. The reddish color of the hands, face, and ears on a cold day demonstrates increased blood flow and vasodilation as a result of low temperatures. To some extent, this cold-mediated vasodilation is useful because it lessens the chance of cold injury to exposed skin. However, if this process included most of the body surface, such as occurs when the body is submerged in cold water or inadequate clothing is worn, heat loss would be rapid and hypothermia would result. (Chapter 29 discusses skin blood flow and temperature regulation.)
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