Dermal capillaries (magnified skin block)

Vasa capillaria dermis

Integumentary (skin)SkinLeg

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What it is

Two horizontal networks of small vessels — a deep plexus at the dermis–hypodermis border and a superficial plexus under the papillae — joined by vertical vessels, with hairpin capillary loops rising into each dermal papilla.

Function

Nourish the avascular epidermis by diffusion and regulate heat loss by opening (flushing) or closing (pallor) the superficial blood flow.

Size

Capillaries ~5–10 µm diameter; papillary loops ~0.2–0.4 mm tall (magnified here)

Clinical note

Skin blood flow can rise from ~0.25 to ~8 litres per minute in heat stress — almost a third of cardiac output — to dump heat.