Physiology

Hair: Hair protects humans from ultraviolet (UV) radiation, minor injury and is of cosmetic value. Hair growth is cyclical and has three phases. Hair growth is usually randomized for individual hairs but may become synchronized during pregnancy. The three phases of hair growth are: anagen, catagen and telogen which are the growth, resting and shedding phases of hair respectively. The normal growth rate for scalp hairs is 0.4 mm/day.

Ultraviolet radiation protection: Melanocytes are located in the basal layer of the epidermis. Melanocytes produce melanin, packaged into granules, which are transfered to adjacent keratinocytes. The melanin granules absorb harmful ultraviolet light.

Temperature Regulation: Maintenance of human body temperature at 37°C is vital to many physiological activities continuously taking place within humans. Temperature regulation depends on metabolism, exposure to outside ambient temperature and weather (with or without protection) and exercise. The skin plays a very important role in controlling body temperature through the direct heat loss from its surface by convection and radiating from blood vessels as well as the loss of heat by the evaporation of sweat.

Environmental Protection: The stratum corneum is important in preventing harmful materials or agents from entering the human body through the skin and also prevents loss of vital body fluids

The skin is also involved in tactile perception, pain perception, and immunobiology in humans. Skin adnexal structures (sebaceous glands, eccrine glands, apocrine glands, hair, etc.) have important physiologic roles in humans and, when diseased, may cause significant morbidity in affected patients.