Human Skin: Structure, Functions and Interesting Facts

The skin is part of the integumentary system that covers the body. The system also includes hair and nails.
The skin is part of the integumentary system that covers the body. The system also includes hair and nails. | Source

An Impressive and Vital Organ

The skin is a very impressive organ that has many vital functions. Skin acts as an enclosure and a barrier, stopping water from entering the body, reducing the loss of water and protecting the body from infection. It also helps to regulate body temperature, produces a vitamin D precursor, protects us from damage by ultraviolet light and detects information in the environment. In addition, the skin contains cells that belong to the immune system and resident bacteria that help us in a variety of ways.

Although skin prevents the entry of water and many other substances into the body, it isn't a complete barrier between the body and the outside world. This is why some medicines can be absorbed through the skin, which is beneficial for us, and why some chemicals in cosmetics can also be absorbed through the skin, which may harm the body. In addition, some of our skin pores allow water to leave the body during perspiration. This process helps us to maintain a constant body temperature.

The skin is the largest organ of the body when we consider both the interior and the surface of the body. The liver is the largest organ inside the body.

The skin is an amazing organ that has vital functions throughout our lives.
The skin is an amazing organ that has vital functions throughout our lives. | Source

Structure of the Skin: An Overview

The skin consists of two layers - the outer, thinner epidermis and the inner, thicker dermis. Underneath the dermis is the hypodermis, also called the subcutaneous layer, which is where fat is stored. The hypodermis isn't considered to be part of the skin, although the bases of the hair follicles and sweat glands may extend into the hypodermis.

The Epidermis

The most abundant cells in the epidermis are the keratinocytes, which are arranged in layers. The keratinocytes in the upper part of the epidermis contain a protein called keratin. Keratin makes the epidermis strong and waterproof. Cells called melanocytes, which produce a protective pigment named melanin, are also present in the epidermis. In addition. Merkel cells, which detect light touches to the skin, and Langerhans cells, which are part of the immune system, are located in the epidermis.

The Dermis

The dermis contains collagen and elastin fibers, hair follicles, sebaceous glands, the coiled sections of the sweat glands, blood and lymph vessels, nerves, sensory receptors and cells from the immune system. The sebaceous glands produce an oily substance called sebum. Most sebaceous glands are connected to a hair follicle.

Attached to each hair follicle is an arrector pili muscle which causes the hair to become erect when the skin is cold or when we experience strong emotions. The erect hairs produce a "goose bumps" or "goose flesh" appearance on the surface of the skin.

Parts of the Skin

A section of human skin
A section of human skin | Source

Resident Bacteria on the Surface of the Skin

It may be surprising to learn that bacteria are an important part of our skin. The bacteria that make their home there are known as resident bacteria, as opposed to temporary visitors which are known as transient bacteria.

Resident bacteria are generally harmless or even helpful. They produce acidic wastes. The bacterial wastes and the lactic acid in our sweat cause the skin surface to have a low pH of around 4 to 5. This pH is fine for the normal bacteria that we carry around but is too low for many harmful bacteria and fungi. Our bacteria population therefore helps to protect us from injury by other microbes. The bacteria may also boost the activity of the immune system in the skin and fight pathogens (microbes that cause disease) in other ways.

Layers of the Epidermis

Source

The epidermis over most of the body is composed of four layers, as shown in the illustration above. The stratum lucidum is present only in thick skin, especially the skin found on the soles of the feet and on the palms.

Keratinocytes and Keratin in the Epidermis

At the base of the epidermis are cells called keratinocytes. These cells divide to make new keratinocytes. The new cells slowly migrate upwards through the epidermis until they reach the top layer, or the stratum corneum. The migration process takes about a month.

As the keratinocytes migrate, they gradually manufacture a chemical called keratin. Keratin is a fibrous protein that forms hair and nails as well as being present in skin cells. It makes the skin tough and contributes to its ability to block water movement through the skin. By the time the migrating keratinocytes reach the surface of the epidermis they have a flattened, hexagonal shape and their keratin is fully formed.

In the stratum corneum the keratinocytes die, although their tough keratin still protects the skin. Eventually the dead cells fall off. This loss is usually balanced by the production of new cells. The cells that leave the body make up a large part of household dust.

Researchers estimate that we lose 30,000 to 40,000 skin cells each minute, or 500 million skin cells per day.

Epidermal Structure

The Epidermis and Vitamin D Production

The process of vitamin D production in the body is a multistep process. The basic steps are follows.

  • A chemical in the epidermis called 7-dehydrocholesterol is struck by ultraviolet light from the sun.
  • The 7-dehydrocholesterol is converted into an inactive form of vitamin D called cholecalciferol.
  • The cholecalciferol is converted into calcidiol in the liver.
  • The calcidiol is converted into calcitriol in the kidneys. Calcitriol is the active form of vitamin D.

Vitamin D is necessary for the absorption of calcium in the small intestine. The calcium is sent to the bones and keeps them strong. The vitamin may also boost the activity of the immune system.

Ultraviolet light from the sun is needed for the skin to make vitamin D, but too much UV radiation can injure the skin.
Ultraviolet light from the sun is needed for the skin to make vitamin D, but too much UV radiation can injure the skin. | Source

Melanocytes, Langerhans Cells and Merkel Cells

Melanocytes

Melanocytes are found in the bottom layer of the epidermis. These cells make melanin, a pigment which gives color to the skin. The pigment is transported to other epidermal cells. Melanin absorbs ultraviolet light, preventing it from damaging the body. It's important to realize that melanin doesn't completely protect us from UV light, however. An additional form of protection is needed when we are exposed to sunlight.

Langerhans and Merkel Cells

The epidermis also contains Langerhans and Merkel cells. Langerhans cells are found in the upper layer of the dermis and in some other body tissues as well as the epidermis. They are classified as a type of dendritic cell because they often have extensions called dendrites. Langerhans cells are part of the immune system, but it's not completely clear how they function. Their biology is an active area of research. Merkel cells are located in the stratum basale close to the dermis. They lie close to nerve endings and are sensitive to light touch.

Other Cells and Chemicals

The dermis also contains other sensory cells as well as a variety of chemicals. These chemicals include lipids and antimicrobial peptides (short chains of amino acids that fight pathogens). The epidermis doesn't contain blood vessels. Nutrients for the epidermal cells are supplied by the blood vessels in the dermis, which also remove waste substances made by the cells.

Langerhans cells in the epidermis during an infection, with a stain added to make the dark granules in the cells clearly visible
Langerhans cells in the epidermis during an infection, with a stain added to make the dark granules in the cells clearly visible | Source

Glands in the Dermis

Sebaceous Glands

The dermis contains three types of skin glands - sebaceous glands, eccrine or merocrine glands and apocrine glands. Sebaceous glands are usually attached to hair follicles. They secrete sebum, an oily substance that contains a mixture of lipids. Sebum lubricates and waterproofs the skin and hair. The greatest amount of sebum is secreted during puberty.

Eccrine Glands

Our skin contains two types of sweat glands, or sudoriferous glands. Eccrine glands are found over most of the body and release sweat directly to the surface of the skin. This sweat is watery and almost odorless. It contains many dissolved chemicals, including water, urea (a waste substance produced from protein metabolism), lactic acid and sodium chloride.

Apocrine Glands

Apocrine glands are found only in certain areas, such as the armpits. They become active at puberty and release a thick, milky and fatty liquid into a hair follicle. Certain conditions, such as stress, stimulate the release of liquid from apocrine glands. When the odorless liquid reaches the surface of the skin, bacteria break it down, producing odoriferous compounds. The function of apocrine glands is unknown. It's been suggested that in the past (and perhaps in the present) their secretion contained a pheromone, which is a chemical that attracts the opposite gender.

The Dermal Layer of the Skin

Collagen and elastin fibers are abundant in the dermis. These proteins give firmness, flexibility and elasticity to the dermis, enabling it to act as a supporting layer for the skin. The dermis also contains sensory receptors and nerve endings to detect temperature, touch, pressure and vibration.

The Skin's Role in Temperature Regulation

The skin has two ways to regulate body temperature. One method is by changing the diameter of the blood vessels. When blood vessels in the dermis dilate, they allow more blood to flow through them. Heat radiates from this blood, moving up through the skin and into the outside world. The reddening of the skin due to increased blood flow can be seen through the thin epidermis. When the body is cold the blood vessels constrict, reducing the flow of blood through the epidermis, making the skin turn pale and reducing heat loss.

The second method of heat regulation is by perspiration. Water leaving the eccrine sweat glands absorbs heat from the skin as it changes into a gas and evaporates into the atmosphere. The gaseous water carries heat from the body with it as it escapes, cooling the body down.

Body Temperature Regulation

Wonderful Skin

Our skin is an amazing organ. It protects us from stresses that could hurt our bodies, helps us to detect our environment and produces important chemicals. We notice changes in our skin's appearance when we're injured or as we age, but many of us don't stop to realize what a marvelous and hard-working structure the skin really is. It's much more than a simple barrier between our body and the outside world.

© 2012 Linda Crampton

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Comments 14 comments

whonunuwho profile image

whonunuwho 4 years ago from United States

I realize that the skin absorbs sunlight and creates vitamin D that we need for bones and teeth. I wonder if vitamin supplements, milk and milk products, or the sun. itself is best in supplying Vitamin D. Sunlight can be a cause of skin cancer and it concerns me about exposing my skin, more so than when I was younger.


AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Hi, whonunuwho. Thanks for commenting. Yes, safe sunlight exposure and an adequate vitamin D supply are very important topics! On the one hand dermatologists tell us that melanin isn't sufficient to protect us from all UV radiation and that we must use sunscreen to protect ourselves from sunburn and skin cancer, and on the other hand researchers say that many of us are deficient in vitamin D, which is turning out to be an extremely important chemical. Deciding whether sunscreen ingredients are safe or not is also a controversial topic!

What I do is use a combination of protective clothing and the safest sunscreen that I can find to protect me from the sun, as well as take a vitamin D supplement every day - but that's another controversial topic. Scientists and nutritionists can't seem to agree on the dose of vitamin D supplement that we should be taking. It needs to be both safe and effective.


drbj profile image

drbj 4 years ago from south Florida

The skin is an amazing, remarkable organ, Alicia, and you have done an excellent job bringing all this valuable, interesting information to your readers. Thank you, m'dear, and a large Up!


AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you, drbj. I appreciate your visit, your comment and the vote! The skin certainly is an amazing organ, and it does so much for us.


mary615 profile image

mary615 4 years ago from Florida

Wow! You really did a lot of research for this Hub. It is very informative and interesting. I guess we just take our skin for granted until something goes wrong like skin cancer. I voted this Hub UP, etc. and I would like to share with followers and FB. May I link this Hub to the one I just published about Basal Cell Carcinoma? Thanks in advance.


AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you very much for the comment and for sharing the hub, mary. I appreciate the link in your hub, too! Skin cancer is a very important topic. I'm glad that you wrote about it.


teaches12345 profile image

teaches12345 4 years ago

I always forget that the skin is an organ. We discussed this in class the other day as we talked about organ donation and this popped up. Interesting topic and well covered.


AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you for the visit and the comment, teaches. It is easy to forget that the skin is classified as an organ!


Peter Geekie profile image

Peter Geekie 4 years ago from Sittingbourne

Dear Alicia C

A very well researched and written article

Thank you

Kind regards Peter


AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you for the comment, Peter Geekie. It's nice to meet you!


girishpuri profile image

girishpuri 4 years ago from NCR , INDIA

Alicia, this is indeed an addition to my knowledge, voted up


AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you for the comment and the vote, girishpuri. I appreciate your visit.


Tonipet profile image

Tonipet 3 years ago from The City of Generals

So enjoyed reading about the wonders of the skin organ. I'm a bit guilty I haven't been giving my skin the best care. Thanks so much for this, Alicia. You are an eye-opener. Blessings! -Tonette


AliciaC profile image

AliciaC 3 years ago from British Columbia, Canada Author

Thank you very much for the visit and the comment, Tonipet!

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    Linda Crampton (AliciaC)1,250 Followers
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    Linda Crampton is a teacher with an honours degree in biology. She enjoys writing about human biology and the science of health and disease.



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