rate reported in the literature is 3.7 liters per hour, measured for Alberto Salazar during the 1984 Olympic Marathon (Armstrong et al., 1986).

If sweat loss is not fully replaced, the individual's total body water will be decreased (dehydration). Because sweat is more dilute than plasma, dehydration from sweat loss results in an increased plasma tonicity and decreased blood volume, both of which will act to reduce sweat output and skin blood flow (Sawka and Pandolf, 1990). As a result, the body's ability to dissipate heat will be decreased, and dehydration will result in a greater rise in core temperature during exercise-heat stress. In addition, the combination of an elevated core temperature and a reduced blood volume will increase the circulatory strain.

Blood flow from the deep body tissues to the skin transfers heat by convection. When core and skin temperatures are low enough that sweating does not occur, raising skin blood flow brings skin temperature nearer to blood temperature, and lowering skin blood flow brings skin temperature nearer to ambient temperature. This phenomenon allows the body to control sensible (convective and radiative) heat loss by varying skin blood flow and thus skin temperature. In conditions in which sweating occurs, the tendency of skin blood flow to warm the skin is approximately balanced by the tendency of sweating to cool the skin. Therefore, there is usually little change in skin temperature and sensible heat exchange after sweating has begun, and skin blood flow serves primarily to deliver to the skin the heat that is being removed by sweat evaporation. Skin blood flow and sweating thus work in tandem to dissipate heat under such conditions.

During exercise-heat stress, thermoregulatory skin blood flow, although not precisely known, may be as high as 7 liters per minute (Rowell, 1986). The higher skin blood flow will generally, but not always, result in a higher cardiac output, and one might expect the increased work of the heart in pumping this blood to be the major source of cardiovascular strain associated with heat stress. The work of the heart in providing the skin blood flow necessary for thermoregulation in the heat imposes a substantial cardiac strain on patients with severe cardiac disease (Burch and DePasquale, 1962). In healthy subjects, however, the cardiovascular strain associated with stress results mostly from reduced cardiac filling and stroke volume (Figure 3-7), which necessitate a higher heart rate to maintain cardiac output (Nadel et al., 1979; Sawka and Wenger, 1988). This change occurs because the venous bed of the skin is large and compliant and dilates reflexively during heat stress. Therefore, as skin blood flow increases, the blood vessels of the skin become engorged and blood pools in the skin, thus reducing central blood volume and cardiac filling.