Skins Circulatory System

Arteries entering the skin arise from more substantial vessels located in the subcutaneous connective tissue. These offshoots form a plexus just beneath the dermis (10). Branches from this subcutaneous network directly supply blood to the hair follicles, the glandular appendages, and the subcutaneous fat. Branches to the upper skin from this deep plexus divide again within the lower dermis, forming a deep subpapillary network. Arterioles reaching the upper dermis out of this plexus are on the order of 50 |im in diameter. They exhibit arteriovenous anastomoses, shunt-like connections that link the arterioles directly to corresponding venules. The dermal arterioles then further branch to form the shallower subpapillary plexus of capillary loops that bring a blood supply up into the papillae at the dermal-epidermal interface. The epidermis itself is avascular.

The veins of skin are organized along the same lines as the arteries in that there are both subpapillary and subdermal plexuses (10). The main arteriole communication to these is the capillary bed. Copious blood is passed through capillaries when the core body is either feverish or overheated, far more than needed to sustain the life force of the epidermis, and this rich perfusion lends a red coloration to fair skin. The vascular surface available for exchange of substances between the blood and the local tissue has been estimated to be of the same magnitude as that of the skin, that is, 1 to 2 cm2 per square centimeter of skin. At room temperature, about 0.05 mL of blood flows through the skin per minute per gram of tissue. This perfusion increases considerably when the skin is warmed (3,11). Sufficient blood courses to within 150 |im of the skin's surface to efficiently draw chemicals into the body that have percutaneously gained access to this depth (6). Blood circulation at this level is turned off by vasoconstrictors (e.g., glucocorticoids) and turned up by vasodilators (e.g., nicotine). These vasoactivities are so reliable that vasoconstriction (blanching) has become an FDA-sanctioned measure of the penetration of corticosteroids through the skin (12,13). The relationships between capillary blood flow and local clearances of percutaneously absorbed drugs, including the influences of vasoconstriction and vasodilation, are not well drawn.

The lymphatic system of the skin extends up and into the papillary layers of the dermis. A dense, flat meshwork of lymphatic capillaries is found here (10). Lymph passes into a deeper network at the lower boundary of the dermis. Serum, macrophages, and lymphocytes readily negotiate through the skin's lymphatic and vascular networks.

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