PHYSIOLOGY OF THE SKIN
The skin is intimately connected with the central nervous system and through it with the other organs and systems.
The skin plays an important part in the metabolic processes which are regulated by the central nervous system. It also plays a significant part in water and mineral metabolism. It is one of the most important water depots of the organism capable of retaining water and large amounts of mineral salts.
The skin participates in the processes of protein, fat and carbohydrate metabolism. Keratinization of the epidermis is associated with complex transformations of albuminous substances and formation of keratohyalin, then eleidin and lastly, keratin. The skin is one of the principal fat depots of the organism. The skin also participates in the metabolism of vitamins А, С and D, and the vitamin В complex (B1, B2 and PP).
The skin is a sense organ. The numerous complex nervous receptors in the skin, connected through nerve fibres with the central nervous system, serve the skin to exercise its function of a sense organ. The "specialized" receptors of the skin receive various stimuli from the external environment and transmit the stimulation to the central nervous system.
Protective Properties of the Skin. As a natural covering of the body the skin protects the organism from various unfavourable external influences — physical, chemical and infectious. The physical influences on the skin are mechanical, thermal (heat and cold), electrical and actinic (the sun, ultraviolet lamps, roentgen rays). The skin very well resists various mechanical influences — friction, shock, pressure,, stretching. The skin owes its strength to its structure — the: density and flexibility of the stratum corneum, the resilient, elastic network of connective tissue fibres of the derma and panniculus adiposus, and the shock-absorbing properties of the loose and resilient fatty lining of the panniculus adiposus. As a poor heat conductor the skin protects the human organism from overheating and cooling in cases of variations in the external temperature and thereby helps to maintain a constant body temperature.
Intact skin protects the organism from penetration of various infectious agents. A healthy skin is impermeable to most pathogenic microbes coming in contact with its surface. The immune properties of the skin also prevent pathogenic microbes from penetrating into the skin and developing therein. The main substance of the derma possesses antimicrobial action. Other protective substances of the type of antibodies have also been discovered in the skin.
Secretory Function of the Skin. The secretory function of the skin is performed by the sweat and sebaceous glands. The sweat glands are a constituent part of man's excretory system. Sweat is a fluid with a low specific gravity (about 1004) and a composition somewhat similar to that of urine. It is 98 per cent water and 2 per cent solid residue. In addition to water the organism excretes through the sweat glands salts (sodium chloride), and protein metabolites (urea, urio acid, ammonia,etc.) Moreover, the secretion of sweat is associated with precesses of heat regulation. Close to 500— 600 ml of sweat is excreted in 24 hours. In cases of hard physical work, high external temperature and fever the amount of excreted sweat may sharply increase (2—4 litres and even more).
The sebaceous glands secrete sebum which serves as an oily lubricant of the skin. About 20 g of sebum is secreted in 24 hours. The fatty lubrication softens the skin, imparts elasticity to the stratum corneum and facilitates the function of contacting surfaces of the skin.
The acid reaction of the sebum hinders the development most of the pathogenic microorganisms о curing on the surface of the skin. Sebaceous glands also participate in the excretory functions of the organism. Certain metabolites are eliminated in the sebum.
Absorptive Ability of the Skin. Healthy, undamaged skin is scarcely able to absorb water, other liquids and solids. Volatile fluids — chloroform, ether, etc.— are absorbed more readily. Solids can be absorbed if theyare dissolved in volatile fluids. Absorption of solids is favoured by their vigorousinunction into the skin. The permeability of normal skin to oxygen, carbon dioxide and water vapours enables the skin to participate in the respiratory function of the organism which consists in absorption of oxygen and elimination of carbon dioxide and water vapours. Thus the skin supplements, as it were, the basic respiratory function performed by the lungs.
Age and Sex Characteristics of the Skin. The structure and function of the skin have certain characteristics associated with age and sex.
The skin of children, especially of infants, contains much more water than that of adults. It is also thinner, more delicate, more vulnerable and more readily macerated than the skin of adults.
In old age the skin undergoes atrophic changes and the secretion of the sebaceous glands diminishes, which is conductive to dryness of the skin.
The atrophic changes in the derma result in a thinning of the skin and loss of its normal elasticity.
In most cases women's skin is thinner and moe delicate. Women usually have less hair on the skin of their extremities and trunk than do men. In women the subcutaneous adipose layer is usually more developed, especially on the hips, buttocks and abdomen, than it is in men.
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