The structure of the skin vasculature differs according to location in the body. In all areas, an arcade of arterioles exists at the boundary of the dermis and the subcutaneous tissue over fatty tissues and skeletal muscles (Fig. 17.5). From this arteriolar arcade, arterioles ascend through the dermis into the superficial layers of the dermis, adjacent to the epidermal layers. These arterioles form a second network in the superficial dermal tissue and perfuse the extensive capillary loops that extend upward into the dermal papillae just beneath the epidermis.
kThe vasculature of the skin. The skin vasculature is composed of a network of large arterioles and venules in the deep dermis, which send branches to the superficial network of smaller arterioles and venules. Arteriove-nous anastomoses allow direct flow from arterioles to venules and greatly increase blood flow when dilated. The capillary loops into the dermal papillae beneath the epidermis are supplied and drained by microvessels of the superficial dermal vasculature.
Sweat glands derive virtually all sweat water from blood plasma and are surrounded by a dense capillary network in the deeper layers of the dermis. As explained in Chapter 29, neural regulation of the sweating mechanism not only causes the formation of sweat but also substantially increases skin blood flow. All the capillaries from the superficial skin layers are drained by venules, which form a venous plexus in the superficial dermis and eventually drain into many large venules and small veins beneath the dermis.
The vascular pattern just described is modified in the tissues of the hand, feet, ears, nose, and some areas of the face in that direct vascular connections between arterioles and venules, known as arteriovenous anastomoses, occur primarily in the superficial dermal tissues (see Fig. 17.5). By contrast, relatively few arteriovenous anastomoses exist in the major portion of human skin over the limbs and torso. If a great amount of heat must be dissipated, dilation of the arteriovenous anastomoses allows substantially increased skin blood flow to warm the skin, thereby increasing heat loss to the environment. This allows vasculatures of the hands and feet and, to a lesser extent, the face, neck, and ears to lose heat efficiently in a warm environment.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.