Review
Skin Blood Flow in Adult Human Thermoregulation: How It Works, When It Does Not, and Why

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The thermoregulatory control of human skin blood flow is vital to the maintenance of normal body temperatures during challenges to thermal homeostasis. Sympathetic neural control of skin blood flow includes the noradrenergic vasoconstrictor system and a sympathetic active vasodilator system, the latter of which is responsible for 80% to 90% of the substantial cutaneous vasodilation that occurs with whole body heat stress. With body heating, the magnitude of skin vasodilation is striking: skin blood flow can reach 6 to 8 L/min during hyperthermia. Cutaneous sympathetic vasoconstrictor and vasodilator systems also participate in baroreflex control of blood pressure; this is particularly important during heat stress, when such a large percentage of cardiac output is directed to the skin. Local thermal control of cutaneous blood vessels also contributes importantly—local warming of the skin can cause maximal vasodilation in healthy humans and includes roles for both local sensory nerves and nitric oxide. Local cooling of the skin can decrease skin blood flow to minimal levels. During menopause, changes in reproductive hormone levels substantially alter thermoregulatory control of skin blood flow. This altered control might contribute to the occurrence of hot flashes. In type 2 diabetes mellitus, the ability of skin blood vessels to dilate is impaired. This impaired vasodilation likely contributes to the increased risk of heat illness in this patient population during exposure to elevated ambient temperatures. Raynaud phenomenon and erythromelalgia represent cutaneous microvas-cular disorders whose pathophysiology appears to relate to disorders of local and/or reflex thermoregulatory control of the skin circulation.

Section snippets

Overview of the Role of the Skin in Human Physiological Thermoregulation

Physiological thermoregulation in humans comprises changes in heat dissipation (cutaneous vasodilation and sweating) and heat generation (shivering) in response to various internal and external thermal stimuli. The central control of thermoregulation is in the preoptic/anterior hypothalamus (PO/AH) in the brain. Information on internal (core) and surface (skin) temperatures is relayed to the PO/AH, which then coordinates the appropriate efferent response.8, 9 Conceptually, this area of the

Reflex Neural Control of Skin Blood Flow via Sympathetic Vasoconstrictor and Vasodilator Nerves

The human cutaneous circulation is unique in that it is controlled by 2 populations of sympathetic nerves. The well-known sympathetic adrenergic vasoconstrictor nerves coexist with sympathetic vasodilator nerves, a less well-understood system that is activated during hyperthermia. Sympathetic vasoconstrictor and vasodilator nerves innervate all areas of nonglabrous skin, whereas areas of glabrous skin (palms, soles, lips) are innervated only by sympathetic vasoconstrictor nerves.1 Another

Skin Blood Flow and Blood Pressure Regulation in Normothermia and Hyperthermia

It was previously believed that the baroreflex controls skeletal muscle, but not skin, blood flow. Indeed, muscle sympathetic nerve activity changes in response to baroreflex stimulation, and skin sympathetic nerve activity does not.53 However, whereas muscle sympathetic nerve activity comprises only vasoconstrictor nerves, skin sympathetic nerve activity includes vasoconstrictor, vasodilator, sudomotor, and piloarrector nerves.1, 3 Therefore, it is difficult to interpret a lack of change in

Local Warming of the Skin

In addition to reflex control of skin blood flow by sympathetic vasodilator and vasoconstrictor systems, the local temperature of an area of skin also contributes importantly to the control of skin blood flow at that site. Local warming of the skin causes a direct and substantial vasodilation in the area being warmed. In healthy humans, a sustained local temperature of 42°C causes maximal dilation of skin blood vessels.59, 60, 61 The vasodilator response to this local warming stimulus is

SKIN BLOOD FLOW AND FEMALE REPRODUCTIVE HORMONES: IMPLICATIONS FOR MENOPAUSE

Menopausal hot flashes are a well-recognized but poorly understood dysfunction of the human thermoregulatory system. Although estrogen deficiency clearly contributes to this phenomenon, the mechanisms involved are unknown. Some insight into this issue comes from studies of the influences of female reproductive hormones on control mechanisms of skin blood flow.15 Both estrogen and progesterone appear to influence skin blood flow control in both young women15, 21, 73 and postmenopausal women.4, 5

SKIN BLOOD FLOW AND TYPE 2 DM

Individuals with type 2 DM appear to be at increased risk for heat illness during exposure to elevated ambient temperatures. There was a markedly increased incidence of heat illness (heat stroke, heat exhaustion) in diabetic patients compared with nondiabetic patients during heat waves in New York, NY, and Chicago, Ill.6, 7 This finding may be linked to cutaneous vasodilator dysfunction and suggests a serious impairment in the ability of these patients to thermoregulate in the heat.

There is

CUTANEOUS MICROVASCULAR DISORDERS

Two important cutaneous microvascular disorders that may be related to altered reflex or local thermoregulation are Raynaud phenomenon and erythromelalgia. Neither disorder is well understood with regard to etiology or patho-physiology.

Raynaud phenomenon affects 3% to 5% of the general population and is characterized by hyperreactive vasoconstriction of the periphery, most often fingers and toes, which can result in pronounced ischemia-reperfusion injury to the skin. Patients with this disorder

SUMMARY

Skin blood flow can reach high levels during thermal stress, approaching 6 to 8 L/min with severe hyperthermia.

Thermoregulatory control of the skin circulation in humans represents a set of physiological control mechanisms that are vital to the maintenance of thermal homeostasis. Despite abundant studies during the past several decades, misconceptions persist regarding some of these mechanisms. The local and reflex thermoregulatory control mechanisms discussed in this review are summarized in

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    This work is supported by grants AR08610 and HL63328 from the National Institutes of Health (NIH), Bethesda, Md, and NIH General Clinical Research Center grant RR00585 (to the Mayo Clinic).

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