Skin

The skin (cutis) covers the outer surface of the body and, in an adult, is an organ measuring roughly 1.6 m2, with an average thickness of 1 to 4 mm. The veins can show as a bluish color through very thin areas of the skin. The skin is a contact organ, meaning that physical events and the environment can come into contact with it. The functions of the skin are outlined in the following paragraphs.

Passiveprotective function:Owing to its strength and elasticity, the skin protects the bodily tissues against harmful influences from outside, such as mechanical effects (eg, knocks, tearing, pressure, and blows), chemical hazards, and heat. However, it also protects against loss of moisture caused by severe evaporation of water from inside the body.

Active protective function: The skin prevents disease-causing organisms from penetrating inside the body; invading infectious pathogens are destroyed by specially stored defensive substances that are partly produced by the skin (acid mantle), or the constant exfoliation of the skin actually makes it difficult for pathogens to penetrate.

Sensory function: There are about half a million touch receptors on the surface of the skin, around a quarter of a million cold receptors, and roughly 30,000 heat receptors. With these sensory nerves, the skin develops excellent surface sensibility in order to enhance the general quality of the body's senses and improve the body's orientation in its surroundings. Sensations can be perceived as pleasant or unpleasant, depending on the strength of stimulus on the different receptors, which cannot objectively reproduce the strength of a stimulus but merely the differences in stimulus. The sensation of pain may also be viewed as a protective function.

Function of heat regulation: The skin radiates heat. The heat given off can be reduced or increased as a result of variations in the blood flow, whereby the skin expands or contracts. The release of heat can be increased as the energy of evaporation is withdrawn due to increased sweat secretion and evaporation of the skin's liquid content. Heat release can also be curtailed when the hairs on the body become erect (gooseflesh) and form an insulating cushion of air.

Secretory function: A considerable proportion of bodily excretions pass through the skin, harmful substances being excreted in the sweat (around 1 liter a day). These harmful substances are mainly sodium chloride, potassium, calcium, and magnesium salts but also include urea and uric acid, lactic acid, and amino acid. As well as these substances, the skin also secretes sebum by means of special glands. Sebum keeps the skin supple. In conjunction with sweat, it forms an emulsion film on the skin, which acts as an acid mantle thanks to the elimination of harmful acidic substances (uric acid and lactic acid). The acid mantle also provides active protection for the skin because it inhibits the growth of bacteria.

Respiratory function: As well as secreting sweat, harmful substances, and sebum, the skin can also give off carbon dioxide. A gas exchange with oxygen also happens because around 1% to 2% of oxygen exchange is regulated by the skin.

Structure

The skin can be divided into three layers, which can be firmly attached to the lower tissue layers by tight bands of connective tissue (Fig 6-38).

The epidermis is the cuticle or outer layer, with its outermost horny layer made up of dead, multilayered squamous epithelial cells. It regenerates constantly and has a good healing tendency. The outer horny layers are constantly being worn away and are replaced by squamous epithelial cells in the underlying basal layer as they divide and work their way to the surface within 28 days.

The dermis (corium) is made up of dense connective tissue of collagen and elastic fibers, in which vascular bundles and nerve endings are located. It is the connecting layer between the epidermis and the subcutis. The interwoven, feltlike connective tissue fibers give this layer of the skin its flexibility, tensile strength, and elasticity.

The subcutis (subcutaneous or subdermal layer) is not strictly classified as belonging to the skin, even though functionally it is part of the skin because it forms the connection with the deeper-lying tissues, such as the tendons or bones. The subcutis is made up of loose connective tissue in which fat cells are lodged. The thickness of the subcutaneous fatty tissue differs depending on where it is located, while the laying down of depot fat is hormonally regulated. The stored fat forms the mantle of fat and the skin's padding of fat.