The outer cover of our body is represented by the skin. The skin has a complex structure and performs important functions:
- the skin is practically impervious to substances and microorganisms;
- strong and elastic skin protects internal organs from mechanical and chemical influences;
- water, mineral salts and other metabolic products are excreted through the skin with sweat;
- skin receptors provide the body’s connection with the external environment;
- the skin performs a thermoregulatory function;
- thanks to the pigment melanin it contains, the skin protects the internal organs from ultraviolet rays;
- it synthesizes vitamin D.
The skin consists of three layers: epidermis, dermis (skin itself) and subcutaneous fatty tissue.
The epidermis is formed by stratified epithelium. The outer stratum corneum is formed by dead cells that constantly slough off. It protects living cells lying deeper from the effects of adverse environmental factors.
The deepest layer of the epidermis is the germ (basal) layer. It reproduces and develops cells replacing exfoliating cells. In this layer, under the influence of sunlight, the dark pigment melanin is produced, the content of which determines the color of the skin.
The dermis, or the skin itself, consists of connective tissue and has a complex structure.
Smooth muscle cells, collagen and elastin fibers give the skin elasticity and strength.
In the dermis, there are numerous nerve endings and tactile, cold, heat receptors that allow us to perceive signals from the environment.
The dermis is riddled with blood vessels. Blood brings oxygen and nutrients, carries away metabolic products.
Sweat and sebaceous glands, hair follicles are located in the dermis.
Sweat glands are glomerular and open on the surface of the skin with long excretory ducts. Sweat contains water, mineral salts, products of protein metabolism (urea, ammonia, uric acid). About 500 cm³ of sweat is usually excreted per day. In hot weather and during physical work, its amount increases to 3 liters. Sweat evaporates from the surface of the skin and cools it.
The mammary glands are also derived from the skin. They consist of 15–20 modified sweat glands that form lobules. The function of the mammary glands is to produce milk for feeding the offspring.
The sebaceous glands resemble vesicles formed by epithelial tissue. The ducts of the sebaceous glands usually open into the hair follicles or to the surface of the skin, where sebum is secreted, lubricating the hair and skin and giving them elasticity.
Subcutaneous adipose tissue is adjacent to the dermis and is represented by loose connective tissue. It contains a large number of fat cells, which accumulate spare fats. The thickness of the layer is different on different parts of the body. This layer acts as a pillow, softens mechanical stress, protects internal organs from injury, and also performs a heat-insulating function.