Thermoregulation and body fluids: role of blood volume and central venous pressure.

Dehydration due to hyperthermia induces both hyperosmolality and hypovolemia. Hyperosmolality reduces evaporative cooling, and alters the thermal responsiveness of the hypothalamic center to changes in both the central and peripheral milieu. Hypovolemia also reduces the thermoregulatory response, but its effect is more variable. The potential sensor of hypovolemia is the CVP, which is influenced by redistribution of blood volume, changes in blood volume, and alterations in cardiac function. The control of CVP is related to the regulation of vascular compliance, stressed blood volume, and unstressed blood volume. Vascular compliance is also involved in the regulation of fluid shifts between the ISF and the intravascular fluid space, and buffers changes in the CVP. Regulation of fluid replacement after thermal dehydration can be considered both from the point of view of osmoregulation and volume regulation. In the rehydration process, control of plasma osmolality precedes blood volume regulation, which also suggests that changes in blood volume sensed as changes in the CVP are regulated within a narrow range by various mechanisms. These findings suggest a hierarchic structure for the homeostatic mechanisms related to thermoregulation, with higher priority being given to the maintenance of cardiac output and the cellular volume of the brain at the expense of peripheral circulation and cell volume.