Thermoregulatory vasodilatation

Cutaneous vasodilatation is one means by which body temperature can be reduced. More blood and its associated heat is transferred from the body core to the periphery of the skin. More blood per unit time through the skin is proportional to more heat loss per unit time from its surface via radiation, conduction and convection.

It is produced by two influences, both acting to reduce precapillary vascular resistance:

• diminshed neural signals from the hypothalamus descending via sympathetic efferent fibres to synapse with alpha-1-adrenoceptors on arteriolar smooth muscle
• local factors e.g. heat, humidity and hypoxia acting on smooth muscle

Thus, blood flow is shunted from deep to more superficial plexuses within the skin. In tandem, the autonomic nervous system may also decrease the volume of blood passing through alternative vascular beds to the skin e.g. the gut, so shunting blood at a more central level.

Finally, a greater volume of blood per unit time through the skin also reduces the efficiency of counter-current exchange mechanisms between arterioles and venules. This results in less heat conservation.