How does the skin work?
The skin is one of our body’s heaviest and largest organs. Depending on body size and mass, it weighs between 3.5 and 10 kilos (7.5 and 22 pounds) and is 1.5 to 2 m2 in size.
The skin protects the body from harmful environmental factors such as dampness, cold and sunlight, but also from germs and harmful substances. It plays an important role in regulating body temperature. It is through our skin that we pick up sensory information: this is how we feel heat, cold, pressure, itchiness or pain. Some of this information triggers a reflex. For example, we automatically pull our hand back if we accidentally touch a hot burner on the stove.
The body can store water or deposit fat and products of metabolism in the skin. This is also the place where essential vitamin D is produced with the help of sunlight. Many health conditions lead to a change in skin color or structure. People with too few red blood cells in their bloodstream may be pale, and hepatitis causes the skin to turn yellow.
If the skin is damaged, more blood flows to the wounded area. That is why the wound is red and warm. Various components of the blood ensure that the wound does not become infected and can heal. Many other substances that are needed for healing are carried to the wound in the blood, particularly oxygen and certain nutrients. Afterwards, cells form to make new skin, subcutaneous tissue and blood vessels. Connective tissue fibers (collagen) and small muscle cells are made too. As a result, the wound becomes more stable and closes. Depending on how deep the wound is, it heals with or without scarring.
The skin consists of three layers: the epidermis, the dermis, and the subcutis.
The outermost, visible layer of skin is called the epidermis. It forms the surface of the skin and constantly rebuilds itself. New cells are made in the lower layers of the epidermis. These move to the surface within four weeks, where they harden and are then shed. Depending on where it is on the body, the epidermis is between 0.03 and 4 mm thick. For example, it is very thin on the forehead, and quite thick on the soles of the feet and palms of the hands. The epidermis contains other cell types as well. Special cells called melanocytes produce and store the pigment melanin. When we lie in the sun, the melanocytes produce more melanin. This makes our skin darker – we get a tan. In this way melanin protects us from the sun’s harmful UV rays. Lymphocytes play an important role in fighting germs. Merkel cells are special nerve cells in the skin that sense pressure.
The dermis (the thick inner layer of the skin) consists of robust, elastic fibers. These ensure that the skin is strong and stable, but also elastic. The dermis has a network of nerve fibers and blood vessels in it. These blood vessels carry nutrients and oxygen to the cells of the dermis and the epidermis, which does not contain any blood vessels. If the skin is stretched a lot, for instance the skin covering the belly during pregnancy, tears in the dermis may result, which can be seen as light lines.
The subcutis (the deepest layer of skin, also called the subcutaneous layer or hypodermis) is mostly made up of fat and connective tissue. The fat acts as a shock absorber and protects bones or joints from external blows or bumps. It serves as insulation too. The dermis and the subcutis also have blood and lymph vessels in them, and other things like nerves, sweat glands, sebaceous (oil) glands and scent glands, as well as the roots of head and body hair.
Author: German Institute for Quality and Efficiency in Health Care (IQWiG)