Heat produced in the body must be delivered to the skin surface to be eliminated. When skin blood flow is minimal, shell conductance is typically 5 to 9 W/°C per m2 of body surface. For a lean resting subject with a surface area of 1.8 m2, minimal whole body conductance of 16 W/°C [i.e., 8.9 W/(°C • m2) X 1.8 m2] and a metabolic heat production of 80 W, the temperature difference between the core and the skin must be 5°C (i.e., 80 W - 16 W/°C) for the heat pro-ducedjo be conducted to the surface. In a cool environment, Tsk may easily be low enough for this to occur. However, in an ambient temperature of 33°C, Tsk is typically about 35°C, and without an increase in conductance, core temperature would have to rise to 40°C—a high, although not yet dangerous, level—for the heat to be conducted to the skin. If the rate of heat production were increased to 480 W by moderate exercise, the temperature difference between core and skin would have to rise to 30°C—and core temperature to well beyond lethal levels—to allow all the heat produced to be conducted to the skin. In the latter circumstances, the conductance of the shell must increase greatly for the body to reestablish thermal balance and continue to regulate its temperature. This is accomplished by increasing the skin blood flow.
Effectiveness of Skin Blood Flow in Heat Transfer. Assuming that blood on its way to the skin remains at core temperature until it reaches the skin, reaches skin temperature as it passes through the skin, and then stays at skin temperature until it returns to the core, we can compute the rate of heat flow (HFb) as a result of convection by the blood as
where SkBF is the rate of skin blood flow, expressed in L/sec rather than the usual L/min to simplify computing HF in W (i.e., J/sec),- and 3.85 kJ/(L-°C) [0.92 kcal/(L-°C)] is the volume-specific heat of blood. Conductance as a result of convection by the blood (Cb) is calculated as:
Of course, heat continues to flow by conduction through the tissues of the shell, so total conductance is the sum of conductance as a result of convection by the blood, plus that result from conduction through the tissues. Total heat flow is given by
in which C0 is thermal conductance of the tissues when skin blood flow is minimal and, thus, is predominantly due to conduction through the tissues.
The assumptions made in deriving equation 8 are somewhat artificial and represent the conditions for maximum efficiency of heat transfer by the blood. In practice, blood exchanges heat also with the tissues through which it passes on its way to and from the skin. Heat exchange with these other tissues is greatest when skin blood flow is low, in such cases, heat flow to the skin may be much less than predicted by equation 8, as discussed further below. However, equation 8 is a reasonable approximation in a warm subject with moderate to high skin blood flow. Although measuring whole-body SkBF directly is not possible, it is believed to reach several liters per minute during heavy exercise in the heat. The maximum obtainable is estimated to be nearly 8 L/min. If SkBF = 1.89 L/min (0.0315 L/sec), according to equation 9, skin blood flow contributes about 121 W/°C to the conductance of the shell. If conduction through the tissues contributes 16 W/°C, total shell conductance is 137 W/°C, and if Tc = 38.5°C and Tsk = 35°C, this will produce a core-to-skin heat transfer of 480 W, the heat production in our earlier example of moderate exercise. Therefore, even a moderate rate of skin blood flow can have a dramatic effect on heat transfer.
When a person is not sweating, raising skin blood flow brings skin temperature nearer to blood temperature and lowering skin blood flow brings skin temperature nearer to ambient temperature. Under such conditions, the body can control dry (convective and radiative) heat loss by varying skin blood flow and, thus, skin temperature. Once sweating begins, skin blood flow continues to increase as the person becomes warmer. In these conditions, however, the tendency of an increase in skin blood flow to warm the skin is approximately balanced by the tendency of an increase in sweating to cool the skin. Therefore, after sweating has begun, further increases in skin blood flow usually cause little change in skin temperature or dry heat exchange and serve primarily to deliver to the skin the heat that is being removed by the evaporation of sweat. Skin blood flow and sweating work in tandem to dissipate heat under such conditions.
Sympathetic Control of Skin Circulation. Blood flow in human skin is under dual vasomotor control. In most of the skin, the vasodilation that occurs during heat exposure depends on sympathetic nerve signals that cause the blood vessels to dilate, and this vasodilation can be prevented or reversed by regional nerve block. Because it depends on the action of nerve signals, such vasodilation is sometimes referred to as active vasodilation. Active vasodilation occurs in almost all the skin, except in so-called acral regions— hands, feet, lips, ears, and nose. In skin areas where active vasodilation occurs, vasoconstrictor activity is minimal at thermoneutral temperatures, and active vasodilation during heat exposure does not begin until close to the onset of sweating. Therefore, skin blood flow in these areas is not much affected by small temperature changes within the thermoneutral range.
The neurotransmitter or other vasoactive substance responsible for active vasodilation in human skin has not been identified. Active vasodilation operates in tandem with sweating in the heat, and is impaired or absent in an-hidrotic ectodermal dysplasia, a congenital disorder in which sweat glands are sparse or absent. For these reasons, the existence of a mechanism linking active vasodilation to the sweat glands has long been suspected, but never established. Earlier suggestions that active vasodilation is cholinergic or is caused by the release of bradykinin from activated sweat glands have not gained general acceptance. More recently, however, nerve endings containing both ACh and vasoactive peptides have been found near eccrine sweat glands in human skin, suggesting that active vasodi-lation may be mediated by a vasoactive cotransmitter that is released along with ACh from the endings of nerves that innervate sweat glands.
Reflex vasoconstriction, occurring in response to cold and as part of certain nonthermal reflexes such as barore-flexes, is mediated primarily through adrenergic sympathetic fibers distributed widely over most of the skin. Reducing the flow of impulses in these nerves allows the blood vessels to dilate. In the acral regions and superficial veins (whose role in heat transfer is discussed below), vasoconstrictor fibers are the predominant vasomotor innervation, and the vasodilation that occurs during heat exposure is largely a result of the withdrawal of vasoconstrictor activity. Blood flow in these skin regions is sensitive to small temperature changes even in the thermoneutral range, and may be responsible for "fine-tuning" heat loss to maintain heat balance in this range.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.