Cutaneous vascular plexuses

Perforating arteries from underlying muscle and fascia form the deep (cutaneous) plexus at the junction of the reticular dermis and hypodermis, supplying hair follicles, sweat glands and fat. Ascending "candelabra" arterioles cross the reticular dermis to form the superficial (subpapillary) plexus at the papillary–reticular junction, from which a single capillary loop rises into each dermal papilla to nourish the avascular epidermis. Venous plexuses run parallel to the arterial ones.

In acral skin — fingers, toes, palms, soles, nail beds, ears, lips and nose — arteriovenous anastomoses connect arterioles directly to venules, bypassing capillaries. Many are coiled, muscular glomus bodies with a heavy sympathetic adrenergic supply. When open, they shunt large volumes of warm blood through the superficial venous plexus to dump heat; when closed, they conserve it. Skin blood flow therefore ranges from near-zero in intense cold to ≈0.3 L/min at rest and 6–8 L/min (up to half the cardiac output) during severe heat stress; the venous plexuses also act as a blood reservoir that sympathetic venoconstriction can mobilise.

Control is predominantly neural: noradrenergic vasoconstrictor fibres (α1/α2, co-released neuropeptide Y) in all skin, plus an active vasodilator system of sympathetic cholinergic nerves (with NO and VIP) in non-acral skin that produces most of the vasodilation during heat stress. Local warming, axon reflexes and inflammatory mediators add local control.