Skin (cutis) - function and structure

Skin (cutis) - function and structure

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The skin - the largest human organ
The human skin is the largest organ in humans - its surface covers up to two square meters. At the same time, there is often a lack of knowledge about how important it is for our lives. Many see it as a kind of shell that holds the inside of the body together.

The skin does that too, but it is much more than a “sack” for bones, flesh and internal organs. As a shell, it separates our body from the outside world, protects it from drying out, keeps pathogens outside as well as sunlight, fends off heat and cold.

The "skin filter" can be used medicinally: creams, oils, lotions, medicinal baths and healing earth applied to the skin provide the body with beneficial substances. Active ingredients such as hormone patches or nicotine patches are also absorbed through them.

A sensory organ

It is also a sensory organ and lets us feel pain as well as temperatures. A network of millions of nerve cells ensures that we can feel whether a surface is smooth or rough, has hair or is made of plant fibers.

With our eyes closed, we use our fingertips to determine a sugar shaker, a chair or a book. Even more: the nerve cells even convey whether the sugar shaker is made of glass or ceramic, has a bulbous or cylindrical shape, whether it is a paperback or a hardcover.

We feel whether the book has a dust jacket and what material it is made of, whether the book contains approximately one hundred or two hundred pages, how high the back of the chair is, whether it is made of wood or metal, whether it is a desk chair or is a living room armchair.

With the senses we not only grasp whether we are touching something, but also what it is. Pain impulses pass the nerves of the outer skin directly into the brain and thus warn us of dangers and possible injuries.

Other creatures such as cats or walruses can even feel things with hair on the facial skin many times better than humans and thus "see" with the skin to a much greater extent.

The skin cells regulate the temperature of the body. A network of vessels and glands ensures that our body heat remains stable.

This superorgan also has a direct effect on our communication and psyche - mostly unconsciously. Shame and anger increase blood circulation and flush our faces.

If we are afraid, we get goose bumps, we feel aggression, the neck hair resists, our fingers twitch when excited.

The skin as a protective shield

It produces sebum like sweat and thus provides a coat that protects us from acid and maintains a pH between 4.5 and 6.9.

Skin protection is not only directed outwards, but also inwards: with sweat, it transports the body's waste to the outside. Conversely, the lipids in the sebum ensure that chemical substances and water are kept away from the inside of the body. At the same time, they ensure that the skin remains sufficiently moist.

This protective shield is vital: if we lose 20% or more of our skin tissue through a fire, we can die from it.

The largest organ

A person of medium height and normal weight is in a skin coat of about two square meters. It is one to two millimeters thick and weighs between three and ten kilograms.

Their color is different for each individual and is due to the amount of blood, the distribution of pigments and the thickness of the epidermis.

Three layers of skin

The skin is divided into the epidermis (epidermis), the dermis (dermis) and the subcutis (hypodermis). The epidermis is primarily a horny layer. It serves as protection to the outside, is constantly renewed and grinds to the outside. The dermis mainly consists of connective tissue and contains the important skin glands. Among other things, the sebum is produced here. The subcutis also mainly contains connective tissue, but this is much looser than in the middle layer and permeated with adipose tissue.

The skin appendages also belong to it. We include hair as well as nails, but also the sweat and sebum glands.

How does the skin protect?

The epidermis is filled with fats. The body loses less water because the fats protect against evaporation. The three layers of skin also provide a buffer zone for blows, bumps or stabs that do not injure the internal organs. The horny layer and the film on the epidermis are also natural sunscreens. They reflect and absorb sunlight. If the rays penetrate deeper, the melanin converts them into heat. The acid protection in sweat and sebum keeps bacteria and fungi out.

The sources of some diseases are already named: If the sun is too strong, the horny layer, skin film and melanin can no longer absorb the rays; If the acid protection is damaged or fungi such as bacteria multiply too much, pathogens can penetrate.

Protection inside

The skin protects the inside of the body by producing antibodies. The epidermis activates the immune system and the body transports blood and lymph to the affected area - everyone knows this from their own skin when it turns red and warms around a wound.

The rash in infections such as measles, rubella or scarlet fever is not a symptom of the disease in the narrow sense, but on the contrary shows how the immune system repels the disease.

Body temperature

Warm-blooded animals depend on a constant body temperature. The skin plays a crucial role in this. The cutaneous vessels contract so that the body does not give off too much heat. That's why we have goose bumps when we freeze. This is because the muscles at the hair follicles contract and the hair straightens up.

Conversely, it also protects against overheating. If the heat builds up in the body, for example during physical exertion or high sunshine, the vessels expand and more heat can leave the body.

It can only fulfill its function as a heat filter to a certain extent. In order to expand this framework, people wrap themselves in an “artificial skin”, clothing. So we can survive outside the (outside) temperatures that our skin controls.

The extent to which we absorb or give off heat via them differs from individual to individual and has to do with genetic differences and skin color. People from cold climates generally have a higher tolerance to cold than people from tropical regions because their skin absorbs more heat and emits less heat. The heat absorption and release can also be trained.

A contact organ

The vernacular describes the skin as a seismometer for the psyche with phrases such as "it gets under my skin" or "I get a rash from it". In fact, it is not only a protection from the outside world, but also an organ to connect with the environment.

The pain receptors are in the dermis, the receptors for pressure in the hypodermis. The thermal receptors collect particularly on the face, lips, chin, nose, ear cups and earlobes. We have almost ten times as many receptors for cold as for heat. It is no coincidence that these are mainly in the areas of the head described: lips, earlobes and the tip of the nose are the first parts of the body to die from excessive cold - the receptors there warn the brain of this danger.

There are also receptors in the dermis that indicate the stretching of the skin.

The receptors for the sense of touch are found in hairless parts, especially in the external genital organs, on the anus, the nipples, the tongue, the fingertips and the lip. The foreskin of the penis has the highest concentration of nerve cells in men. Again, it is no coincidence that the tactile receptors are located at these points: we feel all kinds of objects with our fingertips, we take an early look at the anus to see whether harmful foreign bodies are entering the body, as well as with the lips and tongue.

For example, if we feel the tiny barbs in the skin of a fruit on our lips, it protects us from eating that fruit and possibly damaging the inside of the body. In the genital organs, increased sensitivity through touch promotes sexual arousal.

The color of the skin

The skin color not only differs from individual to individual, but is also clearly shown in different phenotypes of groups of people. These differences promoted the pseudo-scientific theories of human races, which had the primary goal of glorifying or devaluing people of these supposed "races". However, modern biology shows that the skin color arises primarily from an adaptation to the sun's rays and says next to nothing to classify groups of people.

George Chaplin and Nina G. Jablonski put forward the thesis in 2003 that the black and white skin of humans developed as an adaptation to too much and too little sun. This would have been a balancing act. UV rays could have a devastating effect on bare skin cells, and reddish brown to black melanins are natural sunscreens that prevent skin cancer. People with fair skin in regions with strong sun exposure, such as Anglo-Australians, are particularly at risk of skin cancer.

According to the pair of researchers, dark skin was created to protect the folic acid in the body from UV radiation. In the sun-poor north, however, UV-B hardly penetrated them anyway. But that did not bring an advantage, but a problem. Because UV-B rays are dangerous, but they are also vital because they trigger the synthesis of vitamin D and are therefore of fundamental importance for the calcium and phosphate metabolism, which in turn controls the bone structure.

The skin color in the northern latitudes had to be light to absorb enough UV-B rays so that people could produce vitamin D. Without vitamin D, the body cannot absorb calcium from the intestine that makes up the bones, and the skeleton cannot develop normally. Without calcium, the immune system also breaks down.

Michael Hollick of the University of Boston (Massachusetts) and his colleagues have further substantiated these relationships with their medical studies over the past two decades. They also showed that sunlight in higher latitudes in winter is not sufficient for vitamin D production because too little UV-B rays reach the skin cells. So people in the far north would never really get brown. Because your skin should always catch as much sun as possible. On the other hand, people in medium latitudes would become dark in summer and their skin would turn a pale color in winter, in order to save the little sunlight in this season. In summer, their dark skin protects them from too much sun. In the tropics, however, the radiation is so strong that enough vitamin D is also produced with protected pigments.

Inuit in Alaska, Greenland and Northern Canada had darker skin, but only immigrated to the Arctic around 5000 years ago and, secondly, they had largely become independent of the sun: Traditionally, the Inuit ate extremely high-fat marine fish and thus the food the highest levels of vitamin D In Africa, the Khoisan, the Bushmen in southern Africa, had a much lighter skin tone than the Bantu people near the equator, which Chaplin and Jablonski presumably attributed to an adaptation to the lower UV radiation in South Africa.

Today, people often do not adapt to the sun in a new home quickly enough, according to Chaplin and Jablonski. This usually happens out of ignorance. For example, many Indians who came to Britain as Commonwealth citizens in the north of England and Scotland suffered from rickets and other vitamin D deficiency symptoms.

Waste disposal

However, it not only ensures that vitamin D is formed, it also removes table salt (sodium) via sweat. However, like so many things in evolution, it only fulfills this task insufficiently in the mineral balance. Since sweat also serves to cool the body, we not only lose liquid when it is hot, but also salt at the same time and have to replenish it with mineral water, for example.

The surgeon at the entrance

It heals wounds so naturally that we hardly worry about how it happens. If a vessel in the middle layer of skin is damaged, the nerves mediate the injury and platelets fill the affected area. But that's not all: the blood coagulates and simultaneously forms a protein binding agent. This fibrin now settles in the wound as an adhesive and hardens there. A protective layer is created and new skin cells form, finally the wound edges contract and the air dries the initially moist crust.

If the injury is in the lower or dermis, a scar remains. If only the epidermis is injured, everything heals again, if the wound goes into the deeper layers of the skin, a scar can remain. Protective cover and sensory organ, robust and sensitive - our skin is a real marvel.

Oily or dry

Human individuals have either oily or dry skin, or neither. The spectrum is wide. The moisture not only differs from individual to individual, but also changes with age and the body region. For example, adolescents not only have pimples because their sebaceous glands are in full swing, but also often oily hair and oily skin. However, if the sex hormones decrease with age, this process turns around: the skin of old people becomes dry.

Each skin type has its own problems. If it produces too much sebum, the fat thickens the surface, sebum and sweat clog the pores. This in turn allows fungi and bacteria to colonize. Too little sebum, however, means that the important protective film of fat and moisture is only incomplete. The result is that our skin becomes flaky and reacts hypersensitively to cold or dry air. Many people who freeze easily suffer from skin that is too dry - often they do not know about this cause.

When does skin look beautiful?

Skin is considered beautiful if its pores are small, there are no scales, pimples or boils on it, if it evenly reflects the light, contains a little fat, shines and has few wrinkles. Too much fat is just as unaesthetic as dry skin. Our perception can be explained in an evolutionary way: dandruff, pimples and boils can indicate basic illnesses, dry and dull skin firstly on age, secondly also on diseases - or both.

The old skin

Age is not an illness; Our skin ages as young as adults, and it also becomes thinner. We cannot stop this process, but we can mitigate it.

Aging means that the papillae change between the epidermis and dermis. Blood vessels in these papillae supply the epidermis with nutrients, oxygen and fluid. In young people, these papillae are close together and are long - the skin is plump and smooth. In older people, the papillae flatten and become fewer. The older we get, the less collagen and elastin the body forms, and the skin loses its elasticity: we get wrinkles. Nutrients and oxygen now only slowly get into the upper layer of the skin: our skin looks dull. (Dr. Utz Anhalt)

Author and source information

This text corresponds to the specifications of the medical literature, medical guidelines and current studies and has been checked by medical doctors.


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