Body surface temperature: manifestation of complex anatomy and physiology of the cutaneous vasculature

In the thermal balance represented by skin temperature, skin blood flow is a highly variable factor. It needs to be seen in a perspective that encompasses the other major organ vasculatures. High blood flow/mass and blood flow/heat production ratios cause the uniformity of temperatures within the core organs. Low values of these ratios in skin and skeletal muscle underlie the great variations of skin surface temperature, topographically, in a cool environment. But flow can increase enormously in both these organs. In active skeletal muscle, high blood flow brings temperatures close to arterial blood temperature. Even higher flow/mass ratios can develop in the vasculature of skin. Blood bound for skin transits deeper tissues via arterial twigs whose dimensions allow major blood-tissue heat exchange. The vessels of the microvasculature are so near the surface that blood that passes through them comes into near-equilibrium with surface temperature. Variable pathways of venous drainage alter countercurrent exchange potential. Physiologically, human skin is unique in its potential for vasodilation; whether neurally mediated or through direct influence of heat. Both mechanisms lend potential positive feedback instability to skin temperature. Either can call for cardiac output more than double normal. Also important in overall cardiovascular regulation is the high compliance of the cutaneous vasculature that "steals" blood volume from the central venous pool, Finally, skin temperature is an input to reflex control of thermal balance. The remarkable tuning of its influence upon skin blood flow makes this input dominate control within the range of neutral thermal conditions.