Human skin is described as an organ, a membrane, and a system because it functions as all of these things. For instance, the skin is an organ in the integumentary system, which protects the body from harm by preventing damage from outside or leaking from inside. In fact, the skin is the largest organ in the human body. It weighs 10 to 15 pounds, and if you spread it out, it would cover approximately 20 square feet. This varies, of course, according to your height and weight. Although it covers your entire body, its thickness is not uniform. It is thinnest on your eyelids and thickest on the heels of your feet and hands.
According to the online Medical Dictionary, a membrane is a thin layer of tissue that covers a surface. Human skin fulfills this definition by completely covering the surface of the human body. It allows the body to keep its shape and holds all the water and organs that comprise the body inside. Skin has three layers. The outer layer, comprising the first line of defense, is the epidermis. The outermost layer of the epidermis is comprised of dead cells that form a protective barrier. Beneath the epidermis is the dermis, which contains active systems such as nerve fibers, blood vessels, lymph vessels, hair roots, and sweat glands. The third layer, which actually lies below the skin, the subcutaneous, includes a layer of fat that functions as insulation, cushioning, and an energy reserve.
According to the Medical Dictionary, cutaneous means "related to the skin," and skin is also referred to as the cutaneous system. Apart from this reference to skin being a system, the skin also functions as a cooling system. The circulation and regulation of blood under the surface helps cool or warm the body, but especially effective as cooling devices are the sweat glands, which cause perspiration that decreases temperature.