Skin, sweat glands and skin derivatives

The skin is the body’s largest organ and has three layers. The epidermis (outer epithelium) consists of a surface horny layer of dead cells and a dividing layer beneath; melanin pigment in the lower layer protects the skin from the harm of sunlight and gives colour. The dermis (true skin) is connective tissue containing blood and lymph vessels, nerves, receptors (pain, pressure, heat, cold), sweat and sebaceous glands and hair follicles. The subcutaneous fat keeps heat, cushions blows and serves as a reserve. Skin functions: protection (from microbes, light, mechanical effects), sensation, excretion (sweat), a little breathing, making vitamin D and regulating body temperature. In heat vessels in the skin widen, the skin reddens and sweat evaporates, cooling the body; in cold vessels narrow, the skin pales, heat loss falls and muscles shiver to produce heat. Sunlight is useful for vitamin D, but avoid long exposure (risk of burns); in the hottest hours use shade and a hat.