The rapid decrease in plasma volume during stage 2 compromises cardiac filling, stimulating cardiopulmonary volume receptors, which attenuate the vasodilatory reflex and inhibit sweating (Nadel, 1985). Stimulation of the cardiopulmonary receptors also stimulates secretion of antidiuretic hormone (ADH) (Moore, 1971; Segar and Moore, 1968), which reduces free-water clearance and conserves plasma water (Khokhar et al., 1976). ADH may also affect the sweat glands directly, to inhibit sweating (Nadel, 1985). When central blood volume is decreased, arterial blood pressure may fall, stimulating the sinoaortic baroreceptors and thereby causing a redistribution of blood flow away from splanchnic vascular beds (Abboud et al., 1979). This reduction in splanchnic blood volume may be important in conserving plasma water. Horowitz (1984) has demonstrated the importance of restricting splanchnic perfusion for conserving body fluids by comparing the heat stress responses of various species of rats. Since the splanchnic capillaries are among the most porous capillaries of the body to proteins and fluids, a species that can significantly reduce splanchnic blood flow will be most successful in conserving plasma volume and surviving during severe water restriction. Horowitz (1984) reported that the desert rat species Psammomys obesus withstood dehydration for over 48 h at least partly because of its ability to almost completely reduce splanchnic vascular permeability. If such findings can be extrapolated to man, then, reducing splanchnic blood flow during heat exposure is a positive step toward conserving plasma proteins and water.
The increase in plasma osmolality also reduces the rate of plasma volume loss during heat exposure. Sweat is a hypotonic secretion, and therefore, as sweat production continues, the plasma becomes more and more hypertonic. This increase in plasma osmolality inhibits sweating (Fortney et al., 1984) and attenuates the rate of water loss from the vascular compartment.
Aging has been defined as an inability to adapt to changing environmental conditions (Piscopo, 1985). Several investigators (Leaf, 1984; Miller, 1987; Phillips et al., 1984) have observed that elderly individuals have difficulty maintaining body fluid balance. Physiological alterations in water and sodium regulation result in an increased danger of both dehydration and overhydration in the elderly (Crowe et al., 1987). Leaf (1984) observed that nursing home patients have an increased susceptibility to dehydration, and