Skin is an amazingly complex organ and, by weight, the largest of the body. It covers some 22 square feet and weighs around 9 pounds (roughly 7 percent of body weight). Its integumentary system provides the front line of defense for the body, as well as being expressive of physiological conditions and emotional states. Skin is the extension of our nervous system to the outside of the body. Often referred to as our third lung, it is involved in processes of exchange between the internal and external environments -- respiration, absorption and elimination. (Source)