Human skin
Human skin is considered its largest organ. It covers the entire surface of the body and is on average about 2 square meters in size and weighs about 14 kilograms. That's about 20 percent of your body weight. It grows by 0.002 millimeters every day. The skin contains a quarter of the water in the human body, which is about 70 percent water. One square centimeter of skin is made up of around 600,000 cells. It contains 5,000 sensory cells, 4 meters of nerve tracts, 100 sweat glands, 1 meter of blood vessels, 15 sebaceous glands, 5 hairs and 150,000 pigment cells.
The structure of the skin (cutis) consists of three layers that are firmly connected to each other: the upper skin (epidermis), the dermis (corium, dermis) and the subcutaneous layer (subcutis).
The epidermis
The epidermis, the outermost layer of the skin, has no blood supply of its own. It itself consists of several interconnected layers that are constantly renewing themselves. The lowest layer, called the germinal layer or basal layer, is constantly dividing into new basal cells. Within a few weeks, these migrate to the surface of the skin, which is visible from the outside. During their migration they absorb horny substance (keratin) and develop into scale-shaped horny cells (corneocytes). At the end they form the visible stratum corneum. This whole process takes an average of 28 days. The process takes longer for older people than for young people. Up to 14 old horn cells are shed every day.
Depending on the moisture content of the horny layer, the skin is smooth and supple or rough and cracked. It depends on the thickness of the stratum corneum whether the blood vessels shine through and thus make the complexion look pale or rosy. Particularly thick calluses often develop in stressed areas of the body such as the heel, palms or elbows.
Pigment-producing cells (melanocytes) in the germ layer produce the pigment melanin. This is a black-blue pigment that is absorbed into the horny cells of the skin and hair. It determines the color of the skin and hair, which depends on the genetic makeup and the UV radiation absorbed. With high levels of UV radiation, the production of melanin is greater and the skin becomes browner. With darker skin, melanin protects the deeper layers of the skin from UV radiation. UV radiation can damage the genetic information in the cells (DNA).
The germ layer also contains special skin nerve cells and cells of the immune system in order to be able to immediately recognize foreign bodies such as pathogens and start the necessary defense reactions.
The dermis (corium, dermis)
The dermis lies beneath the upper layer of skin. It is a connective tissue that makes the skin tear-resistant and stretchy. It consists of a network of fibers that contain collagen. The thickness of the dermis depends on its load. It is 2.4 millimeters thick on the soles of the feet and is therefore the strongest. It is thinnest on the eyelids at 0.3 millimeters.
The skin's touch receptors, the so-called Meissner touch bodies, are located in the dermis. There are also numerous blood vessels, fatty tissue, hair follicles, nerves and sebum and sweat glands in the dermis. The dermis serves to protect the body from fluctuations in temperature and protects it from mechanical injuries. The dermis also supplies the top layer of skin, which itself has no vessels, with oxygen and nutrients.
The subcutis (subcutis)
Underneath the dermis is the highly stretchable subcutaneous tissue, which consists primarily of loose connective tissue. Extensions of the dermal fibers run through it and are connected to the underlying tissue. Depending on diet, gender and body region, there are different numbers of fat cells in the form of cushions in the connective tissue. These serve as shock absorbers, energy storage and protection from the cold.
The inner skin, the mucous membrane, is often single-layered and is never keratinized. It lines all body cavities such as the mouth, nasal cavity, stomach, intestines, lungs, etc.
The subcutis also contains the hair follicles and special vibration and pressure-tactile bodies, the so-called Vater-Pacini lamellar bodies. The hypodermis is also responsible for allowing the skin to move on the underlying tissues such as the muscles and periosteum.
The function of the skin
The structure of the skin serves to protect the body from harmful substances and pathogens that cause diseases. The skin also protects the body from drying out. In order for the skin barrier to function well, the stratum corneum is particularly important. It is made up of 10 to 20 layers of horny cells. These lie on top of each other like the stones of a wall. Urea, hyaluronic acid and amino acids serve as the mortar.
Healthy, supple skin has a water content of 10 to 20 percent. When the water content of the skin decreases with age or when the skin is subjected to great stress, the surface becomes rough and cracked. Additional moisture is lost through the cracks in the skin and pathogens, allergens and other pollutants can enter the body through them. Dry skin is not just a cosmetic problem, but should also be avoided to keep the body healthy.