The skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs.
Skin performs the following functions:
- Protection: 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: contains a variety of nerve endings that react to heat and cold, touch, pressure, vibration, and tissue injury.
- Heat regulation: the skin contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss while constricted vessels greatly reduce cutaneous blood flow and conserve heat.
- Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to fluid loss. Loss of this function contributes to the massive fluid loss in burns.
- Aesthetics and communication: others see our skin and can assess our mood, physical state and attractiveness.
- Storage and synthesis: acts as a storage center for lipids and water, as well as a means of synthesis of vitamin D by action of UV on certain parts of the skin.
- Excretion: sweat contains urea, however its concentration is 1/130th that of urine, hence excretion by sweating is at most a secondary function to temperature regulation.
- Absorption: Oxygen, nitrogen and carbon dioxide can diffuse into the epidermis in small amounts. In addition, medicine can be administered through the skin, by ointments or by means of adhesive patch, such as the nicotine patch or iontophoresis. The skin is an important site of transport in many other organisms.
- Water resistance: The skin acts as a water resistant barrier so essential nutrients aren’t washed out of the body.
The skin supports its own ecosystems of microorganisms, including yeasts and bacteria, which cannot be removed by any amount of cleaning.