Skin is the soft outer covering of vertebrates. The adjective cutaneous means "of the skin" (from Latin cutis, skin). In mammals, the skin is the largest organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments, and internal organs (Figure 1). The skin is one of the most important parts of the body because it interfaces with the environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, and the production of vitamin D.
Mammalian skin is composed of two primary layers: the epidermis, which provides waterproofing, serves as a barrier to infection, and is a stratified squamous epithelium; and the dermis, which serves as a location for the appendages of skin. The dermis consists of connective tissue and cushions the body from stress and strain.
Skin performs the following functions:
- Protection: Skin acts as an anatomical barrier from pathogens and damage between the internal and external environment in bodily defense. Langerhans cells in the skin are part of the adaptive immune system.
- Sensation: Skin contains a variety of nerve endings that jump to heat and cold, touch, pressure, vibration, and tissue injury.
- Thermoregulation: Eccrine (sweat) glands and dilated blood vessels (increased superficial perfusion) aid heat loss, while constricted vessels greatly reduce cutaneous blood flow and conserve heat. Erector pili muscles in mammals adjust the angle of hair shafts to change the degree of insulation provided by hair or fur.
- Control of evaporation: The skin provides a relatively dry and semi-impermeable barrier to fluid loss.
- Storage and synthesis: The skin acts as a storage center for lipids and water.
- Absorption: Oxygen, nitrogen, and carbon dioxide can diffuse into the epidermis in small amounts. Some animals use their skin as their sole respiration organ. In humans, the cells comprising the outermost 0.25 – 0.40 mm of the skin are almost exclusively supplied by external oxygen, although the contribution to total respiration is negligible.
- Water resistance: The skin acts as a water resistant barrier so essential nutrients aren't washed out of the body. The nutrients and oils that help hydrate our skin are covered by our most outer skin layer, the epidermis. This is helped in part by the sebaceous glands that release sebum, an oily liquid. Water itself will not cause the elimination of oils on the skin, because the oils residing in our dermis flow and would be affected by water without the epidermis.