REFLEX CONTROL OF THE CUTANEOUS VASCULATURE

A major function of cutaneous blood flow is to regulate body temperature. Over most of its great range, skin blood flow far exceeds the metabolic needs of the skin. Cutaneous arterioles and veins are predominantly under neural control from a rich sympathetic adrenergic nerve supply, especially in acral regions. These tonically active, vasoconstrictor nerve fibers are the efferent arm of (1) thermoregulatory reflexes which originate principally in cutaneous thermoreceptors, (2) baroreflexes which originate in both arterial and cardiopulmonary baroreceptors, (3) chemoreceptor reflexes, and (4) reflex responses to upright posture and exercise. Cutaneous vasculature in man is probably unique in its great range and high responsiveness to these reflex drives. Human skin is also unique in possessing a neurogenic vasodilator system that is the principal efferent arm of reflexes which originate in the central thermoreceptors. Apparently, this system participates only in thermoregulatory reflexes; its transmitter is still unknown; and it controls skin blood flow over the full range and over most of the body. In man and baboons, the reflex modification in skin blood flow with each degree (C°) change in central temperature is about 20 times greater than that produced by changing skin temperature. However, in baboons and probably other species, the skin vasomotor system appears to be mainly adrenergic. In man, different reflex drives on skin interact so that the large vasoconstrictor responses (e. g., baroreflex) are still elicited during hyperthermia or direct local heating. Thus, central to peripheral volume displacement is minimized during upright posture, exercise, hemorrhage, etc. despite competing thermal drives. Veins are richly innervated by sympathetic adrenergic nerves which reflexly control their volume