The dermal vasculature is made up of two intercommunicating plexuses: the subpapillary or superficial plexus composed of postcapillary venules found at the junction of the papillary and reticular dermis and the lower plexus at the dermal subcutaneous interface. The dermal papillae are supplied by capillaries, end arterioles, and venules of the superficial plexus. The deeper plexus is supplied by larger blood vessels and is more complex surrounding adnexal structures.
Blood flow in human skin fluctuates significantly in response to thermal stress because of the regulation of the preoptic-anterior hypothalamus. Vasodilation and increased skin blood flow, along with sweating, are crucial to heat dissipation during heat exposure and exercise. During exposure to cold, vasoconstriction in the skin decreases heat loss from the body to prevent hypothermia. Altered control of skin blood flow can considerably impair the ability to maintain normal body temperature. For example, impairments in cutaneous vascular control noted in patients with type II diabetes may contribute to the increased incidence of heat stroke and heat exhaustion during periods of elevated external temperatures. Similarly, menopausal hormones result in the occurrence of hot flashes.