Negative pressure breathing and the control of skin blood flow during exercise in a hot environment

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.

Effect of hydration states on thermoregulatory responses during exercise in rats

Thermoregulatory response during exercise was studied in rats with three hydration states: euhydration, hypohydration (-18% plasma volume by thermal dehydration), and hyperhydration (+24% plasma volume by 5% bovine serum albumin solution infusion). Rats exercised (12.5 m.min-1) for 30 min at an ambient temperature of 25 degrees C. Abdominal temperature (Tab) and heart rate were continuously recorded by using a surgically implanted transmitter and telemetry system, and the tail skin temperature (Tts) was measured as an index of peripheral vasomotor tone. In the euhydration group, Tts showed an increase when Tab reached 37.6 degrees C; an increase of Tts was observed at 37.9 degrees C of Tab in the hypohydration group and at 37.4 degrees C in the hyperhydration group. The slope of the Tts-Tab relationship was steeper in the hyperhydration group and less steep in the hypohydration group than in the euhydration group. These results indicate that hypohydration leads to an upward shift in the body temperature threshold for tail vasodilation and an increase in Tab during exercise. Hyperhydration, however, leads to a downward shift of the threshold and the maintenance of a lower Tab. An elevation of the threshold Tab for vasodilation was observed when plasma volume was reduced by more than 20%, and a decrease was observed when plasma volume increased more than 15%.