induced hemodilution occurs within the first few hours of a controlled laboratory heat exposure. The time course of the changes in plasma volume of men quietly sitting in the heat were described in detail by Bass and Henschel (1956). The hemodilution usually is weak (less than a 5% increase in plasma volume) and transient. Without fluid replacement, it disappears after approximately 30-45 min. It may not appear in subjects who have been dehydrated before the heat exposure (Sawka et al., 1984) or in subjects who have been anesthetized or have suffered transection of the spinal cord below the level of the medulla (Bass and Henschel, 1956). The transient increase in plasma volume during acute heat exposure provides a fluid reservoir for sweat production and attenuates the decrease in central blood volume as sweating continues and as blood flow is redistributed from the core to the skin. Without this initial increase of plasma volume, body temperature would increase more rapidly and heat tolerance would be reduced. With adequate fluid and electrolyte replacement, the expansion of plasma volume persists and plays a key role in reducing cardiovascular strain during the early stages of heat acclimation (Wyndham et al., 1968).
Figure 16-1 illustrates the dynamic plasma volume responses of hydrated healthy young subjects to an acute 3-h heat exposure without fluid replacement. The plasma volume responses can be divided into three stages: initial hemodilution (stage 1), rapid hemoconcentration (stage 2), and slower hemoconcentration (stage 3). Several theories have been offered to explain these plasma volume responses.