Chapter 12. The Integumentary System
Skin makes up the largest part of the integumentary system. Its total surface area in an adult is about 1.5-2 m2. The leading function of the skin is the barrier function, the protection of deeper tissues and organs from mechanical, chemical, physical and other external influences. This function is carried out by various mechanisms. The acidic reaction of the epidermis and the density of its stratum corneum prevent the penetration of microbes into the body. The powerful stratum corneum complicates the absorption and loss of fluid by the body, which is extremely important for maintaining the physiological water-electrolyte balance (in case of skin lesions with extensive burns, rapid dehydration of the body occurs). Substances soluble in lipids (chloroform, ether, phenol) as well as skin-damaging agents (mustard gas, lewisite), on the contrary, penetrate the skin quite easily as they mix well with sebum covering the skin surface and lipid substance located between the cornifield squames of the epidermis. Due to the presence of melanocytes in the epidermis, the skin protects deeper tissues from the adverse effects of ultraviolet rays. The protective function of the skin is also manifested in allergic reactions when exogenous proteins - antigens, enter the body.
In addition to the leading barrier function, the skin plays an important role in protecting the body from hypoand hyperthermia. About 82% of the total heat output of the body passes through the skin surface by convection, radiation, and evaporation. If the thermoregulation function is impaired (for example, during prolonged work in rubber overalls), the body overheats (heat stroke). The skin performs the discharge function - the water loss is about 500 ml a day through the skin via perspiration as well as the elimination of various metabolic products (proteins, chlorides, lactic acid, etc.). The excretory function of the skin is enhanced in diseases of the organs of the excretory system. The skin is involved in immune processes, since it recognizes foreign objects (antigens).
The presence of vascular networks in the skin allows it to play the role of a blood depot - the vessels of the dermis, if expanded, can hold up to 1 liter of blood. The skin is actively involved in the exchange of vitamins. Under the influence of ultraviolet rays, vitamin D is synthesized in epithelial cells, which prevents the development of rickets. The skin is a huge receptor field - in its individual areas per 1 cm2 there are about 200 pain receptors, 25 tactile receptors, and 17 thermoreceptors.
For therapeutic purposes, a close relationship between various topographic areas of the skin and internal organs is used during physiotherapy, acupuncture, massage. For the same reason, the pathological conditions that affect various organs cause hypersensitivity reactions in certain areas of the skin, the Zakharyin-Ged zones. Many skin and infectious diseases are associated with characteristic rashes which play a key role in diagnostic.
The skin forms derivatives, or appendages, in the form of glands (sweat, sebaceous, mammary), hair and nails.