Secretory and excretory functions of the skin

The sweat (eccrine and apocrine) and sebaceous glands are greatly responsible for the secretory and excretory functions of the skin.

The eccrine sweat glands produce sweat which has a weak acid reaction, relative density of 1. 004-1. 008. It consists of 98-99 per cent of water and 1-2 per cent of inorganic compounds such as phosphates, sulphates, sodium and potassium chloride, calcium salts and organic substances (uric acid, urea, creatinine, creatine, ammonium, amino acids, carbohydrates) dissolved in it. The chemical composition of sweat may alter depending on its amount and the organism's general condition. Under normal conditions, the excretion of water by the sweat glands and its evaporation from the epidermal surface is even and imperceptible; this is called insensible perspiration. In the period of increased heat emission, perspiration is visible, profuse, and continuous whereas under normal conditions sweat excretion is pulsatile. Increased perspiration leads at first to copious flow of water from the tissues into the blood, after which the water from the plasma is excreted by the sweat glands. The oral mucosa becomes dry and there is a feeling of thirst.

The apocrine sweat glands, whose function is linked with the endocrine, especially the sex, glands contain in their secretions, besides the common components of sweat, glycogen, cholesterol and its ethers, and iron. Their secretions have a neutral or weak alkaline reaction. They do not play an important role in thermoregulation but, like the sweat glands of the palms and soles, they increase their activity sharply during emotional reactions of the organism.

The sebum, the secretion of the sebaceous glands, has a complex chemical composition. Its main components are free lower and higher fatty acids, neutral fats, nitrous and phosphorous compounds, carbohydrates, various stearins, steroid hormones, and cholesterol compounds. On the surface, sebum mixes with the sweat and forms a fine film of water-fat emulsion. This film plays an important part in maintaining a normal physiological condition of the skin. The excretory function of the sebaceous glands consists in the excretion with the sebum, like with the sweat, of some drugs (iodine, bromine, salicylic acid, antipyrin, etc. ) that had been given to the patients and some toxic substances which form, in particular, in the intestine.

The process of keratinization of the epidermis with gradual conversion of the cell protein substance to keratohyalin, eleidin, and keratin is now considered to be the secretory function of the epidermis.

Respiratory and resorption functions of the skin

The skin takes little part in respiration, i.e. the absorption of oxygen and elimination of carbon dioxide; in children the diffusion of gases through the dilated skin capillaries is more pronounced. As compared to pulmonary exchange, the skin absorbs 1/180 of oxygen and eliminates 1/90-1/65 of carbon dioxide. Besides, the skin discharges water vapours (up to 800 g daily, which is twice to three times the work performed in this respect by the lungs).

Hardly any water or solids are resorbed through the healthy skin, but substances easily dissolved in fats and lipids (resorcinol, sulphur, salicylic and boric acids, lead oxide, ferric chloride, iodine, mercury, chloroform, pyrogallol, etc.) are readily resorbed by it. The degree of resorption on a skin area depends on the condition of the water-lipid mantle, the buffer capacity of the skin surface, and the presence of the sebaceous follicles. Therefore the skin of the palms and soles, for example, which is devoid of sebaceous glands, has low resorption capacity, the more so since there is a dense corneal layer.

In some cases, however, resorption is intensified. Factors conducive to this are swelling of macerated epidermis, long-term rubbing into the skin of finely ground substances, application to the skin of solids dissolved in volatile fluids (e.g. salicylic acid in ether), hyperemia of the skin.