Sweat glands, also formed from invaginations of the epidermis, are embedded in the middle of the dermis. Their ducts penetrate the epidermis to the outside and excrete sweat. Sweat is composed of 98 percent water and two percent dissolved salts and nitrogenous wastes, such as urea and uric acid. Capillary blood vessels and nerves, including those that serve the skin's muscles, sweat glands, and blood vessels as well as those that serve as sensory receptors, pervade the entire dermis but not the epidermis. Located under the dermis is the layer of very loose connective tissue that joins the skin to the body. This layer, not technically part of the skin, houses many fat cells.
Sweat glands and the capillary blood vessels of the skin function together to help the body thermoregulate itself. When the body is too warm, the flow of perspiration from the sweat glands to the ducts to the surface of the skin increases. As the perspiration evaporates, heat is removed from the surrounding tissues in order to change the liquid to a gas, thus producing a cooling effect on the body. At the same time, the capillaries in the skin dilate, which increases the flow of blood to the skin. Because blood rapidly loses heat by radiation to the outside, the greater the blood flow to the skin, the greater the heat lost. Conversely, when the body is too cool, the capillaries constrict, thus reducing blood flow to and heat loss from the skin.
