Functions




Skin performs the following functions:

  1. Protection: an anatomical barrier from pathogens and damage between the internal and external environment in bodily defense. (See Skin absorption.) Langerhans cells in the skin are part of the adaptive immune system.
  2. Sensation: contains a variety of nerve endings that jump to heat and cold, touch, pressure, vibration, and tissue injury (see somatosensory system and haptic perception).
  3. 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.
  4. Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.
  5. Storage and synthesis: acts as a storage center for lipids and water
  6. Absorption through the skin: 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") Some medications are absorbed through the skin.
  7. 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 the skin are covered by the 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.
  8. Camouflage, whether the skin is naked or covered in fur, scales, or feathers, skin structures provide protective coloration and patterns that help to conceal animals from predators or prey.

Mechanicsedit

Skin is a soft tissue and exhibits key mechanical behaviors of these tissues. The most pronounced feature is the J-curve stress strain response, in which a region of large strain and minimal stress exists and corresponds to the microstructural straightening and reorientation of collagen fibrils. In some cases the intact skin is prestreched, like wetsuits around the diver's body, and in other cases the intact skin is under compression. Small circular holes punched on the skin may widen or close into ellipses, or shrink and remain circular, depending on preexisting stresses.

Ageingedit

Tissue homeostasis generally declines with age, in part because stem/progenitor cells fail to self-renew or differentiate. In the skin of mice, mitochondrial oxidative stress can promote cellular senescence and aging phenotypes.citation needed Ordinarily mitochondrial superoxide dismutase (SOD2) protects against oxidative stress. Using a mouse model of genetic SOD2 deficiency, it was shown that failure to express this important antioxidant enzyme in epidermal cells caused cellular senescence, nuclear DNA damage, and irreversible arrest of proliferation of a fraction of keratinocytes.

Skin aging is caused in part by TGF-β, which reduces the subcutaneous fat that gives skin a pleasant appearance and texture. TGF-β does this by blocking the conversion of dermal fibroblasts into fat cells; with fewer fat cells underneath to provide support, the skin becomes saggy and wrinkled. Subcutaneous fat also produces cathelicidin, which is a peptide that fights bacterial infections.

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