How does skin work?
Even at its thickest point, our skin is only a few millimeters thick. But it is still our heaviest and largest organ, making up about one seventh of our body weight: Depending on your height and body mass, it weighs between 3.5 and 10 kilograms (7.5 and 22 pounds) and has a surface area of 1.5 to 2 square meters. This goes to show how important skin is for your body and metabolism.
What does skin do?
Skin has a lot of different functions. It is a stable but flexible outer covering that acts as barrier, protecting your body from harmful things in the outside world such as moisture, the cold and sun rays, as well as germs and toxic substances.
Just looking at someone’s skin can already tell you a lot – for instance, about their age and health. Changes in skin color or structure can be a sign of a medical condition. For example, people with too few red blood cells in their blood may look pale, and people who have hepatitis have yellowish skin.
Skin also plays an important role in regulating your body temperature. It helps prevent dehydration and protects you from the negative effects of too much heat or cold. And it allows your body to feel sensations such as warmth, cold, pressure, itching and pain. Some of these sensations trigger a reflex, like automatically pulling your hand back if you accidentally touch a hot stove.
Skin also functions as a large storeroom for the body: The deepest layer of skin can store water, fat and metabolic products. And it produces hormones that are important for the whole body.
If skin is injured, the blood supply to the skin increases in order to deliver various substances to the wound so it is better protected from infections and can heal faster. Later on, new cells are produced to form new skin and blood vessels. Depending on how deep the wound is, it heals with or without a scar.
To be able to do all of these things, skin consists of three different layers: the outer layer (epidermis), the middle layer (dermis) and the deepest layer (subcutis). Depending on where it is on your body and the demands made on it, your skin varies in thickness. The thickness of your skin depends on your age and sex too: Older people generally have thinner skin than younger people do, and men generally have thicker skin than women do.
The outer layer (epidermis)
The outermost layer of skin which you can see is called the epidermis. It is mostly made up of cells that produce keratin (keratinocytes). These cells are gradually pushed to the surface of the skin by newer cells, where they harden and then eventually die off. The hardened keratinocytes (corneocytes) are packed closely together and seal the skin off from the outside environment.
The epidermis constantly renews itself: New cells are made in the lower layers of the epidermis. These move to the surface within four weeks. This constant renewal serves to replace the cells that are lost and fall to the ground as tiny flakes of skin when the skin is rubbed. The cells in the epidermis grow faster in response to pressure or rubbing. The amount of skin flakes that are shed remains the same, though. As a result, the layer of hardened skin on the surface gradually becomes thicker and a callus develops. The skin does this to protect itself – to better withstand pressure and rubbing.
Only rarely is the balance of new cell production and old cell shedding affected by illness. Examples include infections, autoimmune disorders or genetic diseases that cause increased growth of rough, scaly skin on the entire body. Hardened skin in only one place may be a sign of non-melanoma skin cancer or skin changes that may develop into cancer.
Depending on where it is on the body, the epidermis varies in thickness. For instance, it is only 0.3 millimeters thick on your elbows and the back of your knees, and up to 4 millimeters thick on other parts of your body such as the soles of your feet and palms of your hands.
The epidermis also contains other types of cells with special functions:
- Melanocytes produce and store a black pigment called melanin. They produce more melanin when your skin is exposed to sunlight, which is why it becomes darker. This protects the skin from the sun’s harmful UV rays.
- Lymphocytes and Langerhans cells play an important role in fighting germs. They “grab” the germs and take them to the nearest lymph node.
- Merkel cells are special nerve cells in the skin that enable you to sense pressure.
The middle layer (dermis)
Under the epidermis, firmly stuck to it, lies the middle layer of skin (the dermis). It is made up of a dense network of tough, elastic collagen fibers. These make the skin strong and robust, while at the same time stretchy. If skin is stretched a lot – for instance the skin covering a pregnant woman’s belly – the dermis might tear. The torn dermis can be seen as light lines (stretch marks).
In places, the dermis bulges into the connective tissue that surrounds our muscles and bones and connects them with the skin.
The dermis contains a network of nerve fibers and very small blood vessels called capillaries. Nutrients and oxygen in the blood pass from the capillaries into cells. The other main function of the capillaries is to help your body cool down if it gets too hot. The dermis is also the skin layer that contains the most sensory (feeling) cells and sweat glands.
The deepest layer (subcutis)
The subcutis (also known as the subcutaneous layer or hypodermis) is mostly made up of fat and connective tissue. In the subcutis, between the folds of dermis that bulge into it, there are tiny cavities. These cavities are filled with storage tissue made out of fat and water. The fat acts as a shock absorber, protecting bones and joints from blows or bumps. It serves as insulation too. What’s more, many hormones are produced in the fat cells of the subcutis. One example is vitamin D, which is an essential vitamin and is made when the skin is exposed to sunlight.
The subcutis and dermis both contain blood vessels and lymph vessels too, as well as other things like nerves, sweat glands, sebaceous (oil) glands, scent glands and hair roots.
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