Microvascular volume changes induced by exercise, heat exposure, or endotoxin injection.

The microcirculation as a blood reservoir for volume compensation and the microvascular volume change (MVC) as a means of blood volume regulation are investigated. The density equations of LaForte et al. [Am. J. Physiol. 262 (Heart Circ. Physiol. 31): H190-H199, 1992] are modified to assess the MVC and volume of fluid restitution from changes in systemic hematocrit and plasma protein concentration induced by maximal exercise. It is estimated that exercise causes an MVC that is 5.6% of total blood volume (Vb) to counter the effect of plasma volume loss at 9.4% of Vb. A similar analysis of the result for 120 min of heat exposure indicates that the plasma volume decreases by 8.1% of Vb while the microcirculation constricts to produce an MVC that is 7.9% of Vb, with a net result of insignificant change in the macrovascular volume. Analysis of endotoxin injection to splenectomized dogs indicates that the microvascular dilation and plasma volume decrease combine to decrease the macrovascular volume by 31%, which may lead to the endotoxin shock. I conclude from these findings and the results of lower body negative pressure suction and cyclic hemorrhage that MVC could alter macrovascular filling to change cardiovascular function. It is also shown that the use of systemic hematocrit change as the sole determinant of plasma volume change could be erroneous.