Sunlight and the Skin

Skin Function and Structure

The skin is largest organ of the human body, in terms of external surface area, covering as it does the whole of the body’s surface. An average person has a skin area of about 1.5–2.0 square metres (16–21 sq. ft). It is, however, very thin, at around 2–3 mm (0.1 in) thick in most places.

The human skin performs several important functions:

  • Protection against physical damage, harmful chemicals and infection. It also provides a water-resistant barrier to prevent vital nutrients being washed out of the body or lost through evaporation.
  • Regulation of body temperature through control of blood flow through the numerous tiny blood vessels of the skin. Greater blood flow increases heat loss through the skin by radiation, conduction and convection and cools the blood; reduced blood flow conserves body heat. Sweating also assists heat loss through evaporation.
  • Sensation of touch, temperature, pressure, vibration and tissue injury are all provided by the numerous nerve endings of various types in the skin.
  • Production of vitamin D occurs in the skin in the presence of UVB radiation; this is the body’s primary source of this important vitamin, since it is rarely found in foods.
  • Communication of mood and physical state to others can be assisted by visible changes in the skin.

The skin is divided into two main layers: the epidermis (nearest the surface) and dermis. Below the dermis lay the subcutaneous tissues (hypodermis) that attach the skin to the underlying muscles and bones, contain the veins and arteries that supply the smaller vessels in the skin itself, and also store fat (the hypodermis contains about 50% of the body’s fat).

The Epidermis

The epidermis is itself divided into several layers, or strata, and contains three main types of cells:

  • Squamous cells: flat cells that form a thin layer at the surface of the skin.
  • Basal cells: more rounded cells that form the bulk of the epidermis.
  • Melanocytes: special cells that occur between the basal cells. It is these cells that are responsible for the production of melanin, the pigment that gives the skin its light/dark colour variations: the more melanin, the darker the skin colour.

The epidermis is continually being regenerated, with new cells being formed at the bottom which then gradually move upwards, changing shape and composition, becoming filled with keratin, until they reach the surface and are lost. The cells gradually die as they rise through the epidermis and become separated from their food supply: there are no blood vessels in the epidermis. The surface of the epidermis consists of 25–30 layers of dead cells that have become hardened with keratin and form the main, tough protective outer layer of the skin. The whole process typically takes 52–75 days, becoming faster with certain diseases, such as psoriasis.

The Dermis

The dermis forms the bulk of the thickness of the skin, being several times thicker than the epidermis, and is divided into two layers, or regions: a thin papillary region, next to the epidermis, and the much thicker reticular region underneath.

The papillary region connects the epidermis and dermis together, and is so called because of the numerous, tiny finger-like projections (papillae) that extend into the epidermis and strengthen the connection. It is these papillae that give rise to the ridges in the surface of the epidermis on the palms, fingers, soles and toes that form the fingerprints (and toeprints) and improve grip by increasing friction.

The reticular region is named after the many elastic, collagenous reticular fibres that are distributed throughout it and give the dermis its strength, extensibility and elasticity. The dermis also contains many other important structures:

  • Many tiny blood vessels distributed throughout the dermis provide nourishment and waste removal both for it and the base of the epidermis.
  • Many nerve endings that provide the senses of touch and heat. Some nerves also extend into the epidermis.
  • Sweat glands with associated ducts through to the surface of the epidermis. These are somewhat unevenly distributed, with the highest densities occurring in the palms of the hands and soles of the feet.
  • Hair follicles are located at the bottom of the dermis, and the hairs grow through to the surface of the epidermis.
  • Sebaceous glands are usually located adjacent to the hair follicles, and the sebum (oil) they produce is conducted to the surface along the hairs. They are also found in hairless areas, when their sebum reaches the skin surface through ducts similar to those for sweat glands. The sebum protects and waterproofs the hairs and skin, preventing them becoming dry and brittle.
  • Apocrine glands are specialised sweat glands that release their secretions into the hair follicles of the armpits, groin and around the nipples. Their secretions are more viscous and potentially odorous than normal sweat, and may also contain pheromones.
  • Many tiny lymph vessels are distributed throughout the dermis and serve to drain it of excess fluids that are returned through the lymph system to the blood.

On average, each square half inch of skin contains 10 hairs, 15 sebaceous glands, 100 sweat glands and 1 m (3.2 ft) of blood vessels.

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