Thermoregulation and Body Fluid Osmolality

Thermoregulatory responses induce dehydration, and dehydration itself raises body temperature, causing an increase in the threshold temperature for cutaneous vasodilatation and sweating, the sensitivity of cutaneous vasodilatation in response to a unit rise in body temperature, and the maximum attainable level of cutaneous circulation, and sweat rate. The reduction of these thermoregulatory responses has been related to hypovolemia and hyperosmolality. Evidence showing the involvement of cardiopulmonary baroreceptors is discussed along with an introduction on the effect of hyperosmolality on skin blood flow and sweating and the involvement of central nervous mechanisms. Heat induced hyperosmolality triggers regulatory responses maintaining blood volume and circulatory function, including a fluid shift between body fluid compartments and the control of fluid intake. Evidence showing the importance of the osmotic regulation of body fluid by drinking is also presented. Finally, the effect of hypovolemia and hyperosmolality under thermal stress due to hot environment or physical activity is discussed from the viewpoint of the interaction between circulation, thermoregulation and body fluid homeostasis.