Hypoxic regulation of blood flow in humans. Skin blood flow and temperature regulation.

Regulation of cutaneous vascular tone in humans is complex due to the different types of skin in various regions of the body and the vast array of nerves involved in regulation of blood flow. Due to these complexities, it is unclear how the cutaneous vasculature responds to hypoxia. There are reports of exaggerated vasoconstriction and vasodilation, while others suggest the skin is unresponsive to a hypoxic stimulus. Preliminary work in our laboratory suggests hypoxic vasodilation may be unmasked with alpha-receptor blockade. In contrast to skeletal muscle, hypoxic cutaneous vasodilation is not blunted by beta-blockade, but may be abolished with NO-synthase inhibition. Furthermore, effects of hypoxia on skin blood flow may be more pronounced during combined hypoxic and thermoregulatory challenges. Along these lines, overall thermoregulation may be impacted by hypoxic effects on the cutaneous vasculature and hypobaric effects on sweating and evaporation. During supine heat stress, for example, skin blood flow can increase to 8 Liters per minute. This dramatic rise in skin blood flow is accomplished by an increase in cardiac output and redistribution of blood flow from the splanchnic and renal vascular beds. During hypoxia, splanchnic blood flow has been shown to increase. Thus, during a hypoxic challenge in the heat, a competition for blood flow between the compliant skin and splanchnic regions must exist, but is not well understood. In this review, the effects of hypoxia on the regulation of cutaneous vascular tone and the impact on temperature regulation will be discussed.