The skin is the largest organ in the body. It weighs on average 4 kg and covers an area of approximately 2 m2. The two major functions of the skin are as a barrier protecting the body from the external environment and as a thermoreg-ulatory organ. As a consequence, in addition to providing nutritional support and maintaining tissue homeostasis, the skin microvasculature also plays key roles in immunosur-veillance, hemostasis, and tissue repair and remodeling, as well as in thermoregulation. Consequently, the small blood vessels supplying the skin have become highly organized into two horizontal plexuses (Figure 1), the upper of which lies 1 to 1.5 mm below the epidermis. This superficial vascular plexus forms the dermal papillary loops that are the major site of exchange of gases and nutrients within the skin. The three-dimensional structure of this superficial plexus has been extensively studied and painstakingly reconstructed by Braverman and his colleagues . Indeed, because these vessels are the primary site of anatomical variation in a number of skin diseases, it is one of the best characterized vascular beds both in health and disease. The lower plexus is located on the dermal-hypodermal junction. These vessels supply the upper plexus and also branch to supply dermal appendages, such as hair follicles and sweat glands.
In acral or nonhairy skin, the two plexuses are additionally connected by arteriovenous anastomoses—important in the thermoregulatory shunting of blood between the upper and lower plexuses. These anastomoses, which are approximately 30 to 35 mm in diameter, occur mainly in the periphery at exposed sites such as the fingers and toes, lips, nose and ears. As much as 60 percent of the cutaneous blood can be shunted through these anastomoses away from the capillary loops and to the venous plexus in order to reduce con-vective heat loss.
As a result of the need to accommodate the many different and sometimes conflicting requirements of the tissue, blood flow in the skin is extremely variable. In a ther-moneutral environment, flow generally ranges between 10 and 20mLmin-1 100g-1. It may, however, fall as low as <1mLmin-1 100 g-1 or reach maximal flows of 150 to 200mLmin-1 100 g-1 at times of cold or severe heat stress, respectively. Not unexpectedly, control of this labile vascular bed is complex and involves neural, humoral, and local influences. Together this affords the investigator a rich opportunity to investigate the functional regulation of microvascular perfusion and integrity.
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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.