Many conditions affect the human integumentary system —the organ system covering the entire surface of the body and composed of skin , hair , nails , and related muscle and glands . The major function of this system is as a barrier against the external environment. The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis , dermis , and subcutaneous tissue . The two main types of human skin are: glabrous skin , the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, the hairs occur in structures called pilosebaceous units , each with hair follicle , sebaceous gland , and associated arrector pili muscle. In the embryo , the epidermis, hair, and glands form from the ectoderm , which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.