Responses to hyperthermia. Optimizing heat dissipation by convection and evaporation: Neural control of skin blood flow and sweating in humans

Abstract

Under normothermic, resting conditions, humans dissipate heat from the body at a rate approximately equal to heat production. Small discrepancies between heat production and heat elimination would, over time, lead to significant changes in heat storage and body temperature. When heat production or environmental temperature is high the challenge of maintaining heat balance is much greater. This matching of heat elimination with heat production is a function of the skin circulation facilitating heat transport to the body surface and sweating, enabling evaporative heat loss. These processes are manifestations of the autonomic control of cutaneous vasomotor and sudomotor functions and form the basis of this review. We focus on these systems in the responses to hyperthermia. In particular, the cutaneous vascular responses to heat stress and the current understanding of the neurovascular mechanisms involved. The available research regarding cutaneous active vasodilation and vasoconstriction is highlighted, with emphasis on active vasodilation as a major responder to heat stress. Involvement of the vasoconstrictor and active vasodilator controls of the skin circulation in the context of heat stress and nonthermoregulatory reflexes (blood pressure, exercise) are also considered. Autonomic involvement in the cutaneous vascular responses to direct heating and cooling of the skin are also discussed. We examine the autonomic control of sweating, including cholinergic and noncholinergic mechanisms, the local control of sweating, thermoregulatory and nonthermoregulatory reflex control and the possible relationship between sudomotor and cutaneous vasodilator function. Finally, we comment on the clinical relevance of these control schemes in conditions of autonomic dysfunction.

Effects of environmental temperature on physiological responses during submaximal and maximal exercises in soccer players

“…Furthermore, as the present study showed, environmental temperature conditions may be an important factor to regulate endurance exercise capacity, 26 emphasizing the interaction between the body's mechanisms for heat balance and environmental conditions. During heat stress, vasodilation occurs in skin blood vessels so that the heat is lost from the skin and sweat glands become more active to increase evaporative heat loss 3 . In contrast, during cold stress, vasoconstriction occurs in skin blood vessels so less heat is lost to the environment and skeletal muscle shivering is activated to generate metabolism and heat 2 …”

“…There is a growing body of evidence indicating that the effects of both hyperthermia in the heat and exercise stress may be associated with the central nervous system 3 . The central nervous system seems to be critical during exercise in hot environments.…”

“…Also, dynamic exercise increases stroke volume and oxygen uptake11, 12 and increases blood flow to the skeletal muscle and skin, thereby promoting body heat loss 3 . However, exercise with prolonged and/or repeated cold or heat exposure may impair thermoregulatory control such as shivering and vasoconstriction responses to cold, and evaporation and vasodilation responses to heat.…”

Nicotinic receptors modulate skin perfusion during normothermia, and have a limited role in skin vasodilatation and sweating during hyperthermia

“…During whole‐body heating, cutaneous vascular conductance gradually increased (Figure ), which is indicative of heat‐induced cutaneous vasodilatation (Johnson et al., ; Smith & Johnson, ; Wong & Hollowed, ). We found that the reduced cutaneous vascular conductance associated with the administration of hexamethonium observed under normothermic resting states was gradually diminished as oesophageal temperature increased by >0.4°C above baseline resting levels (Figure , all P ≥ 0.12).…”

“…An increase in whole‐body heat loss is achieved by the activation of the heat loss responses of cutaneous vasodilatation and sweating. However, the mechanisms responsible for the activation of heat loss responses are complex and involve the interplay of different factors that modulate the response (Johnson, Minson, & Kellogg, ; Smith & Johnson, ; Wong & Hollowed, ). Of these, sympathetic cholinergic nerves have been shown to play a critical role in the regulation of cutaneous vasculature and sweating responses as evidenced by the observation that presynaptic cholinergic nerve blockade with botulinum toxin type A abolishes the heat‐induced activation of cutaneous vasodilatation and sweating (Kellogg et al., ).…”