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Integumentary system -

The human skin is the outer covering of the body. It's our largest organ and is part of the integumentary system—the skin and its appendages. Pictured is a 3D image showing the complex system.

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Three layers of skin -

Skin is composed of three primary layers: the epidermis, the dermis, and the hypodermis. The epidermis is the outermost layer of skin. This is our waterproof, protective wrap. Beneath that is the dermis, which cushions the body from stress and strain. The third layer, the hypodermis, is used mainly for fat storage.

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A lot of skin -

The average person's skin covers an area of two square meters, or 22 square feet.

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Hair -

The main skin cell that makes up the epidermis is called the keratinocyte, named as such because it produces a tough protein called keratin, which is responsible for hair growth.

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Nails -

Keratin is also the protein from which nails are formed.

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Helps maintain equilibrium -

The integumentary system has multiple roles in maintaining the body's equilibrium. Indeed, the skin has an important job of protecting the body's internal living tissues and organs. Keratin is key because it provides skin with much of its resistance to physical wear and tear. It also makes skin waterproof.

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First line of defense -

Skin is one of our body's first lines of defense against harmful microbes. Immune cells within skin tissue help ward off invading organisms.

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Protection against dehydration -

Our skin helps protect us from dehydration. How, exactly? The epidermis and dermis, together with sweat glands and hair follicles, retain moisture and nutrients, and regulate water. Again, it's the waterproof nature of our skin that helps us keep hydrated.

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Homeostasis -

Our bodies are protected from abrupt changes of temperature by maintaining homeostasis, the self-regulating process by which the body remains stable while adjusting to conditions that are best suited for our survival. The functions of skin in homeostasis include protection, regulation of body temperature, sensory reception, water balance, and synthesis of vitamins and hormones.

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Sweat it out -

The skin helps excrete waste materials through perspiration. Sweating eliminates excess water and salts, and other metabolites from the bloodstream, including urea, uric acid, and proteins.

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Skin sensations -

Our sense of touch is controlled by a vast network of nerve endings and touch receptors in the skin known as the somatosensory system. In fact, this system is responsible for all the sensations we feel—hot, cold,  pressure, pain, etc.

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A network of receptors -

Actually, your skin has at least five different types of receptors that respond to pain and touch.

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Ultraviolet rays -

We all know the risks of too much sun. And the body has a way of protecting the skin, at least up to a point, with melanin. This natural pigment found in the epidermis protects skin from the sun's harmful ultraviolet (UV) rays.

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Skin absorbs sunlight -

While too much sun is detrimental to your health, too little can also have adverse effects on the body. The skin absorbs sunlight to generate vitamin D. It's often called the "sunshine vitamin" for this very reason. The sun's ultraviolet B (UVB) rays hit cholesterol in the skin cells, providing the energy for vitamin D synthesis to occur.

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A natural storeroom -

Skin serves as an ideal storeroom for the body. The deepest layer of skin, the hypodermis, can store water, fat, and metabolic products such as glucose. The hypodermis also produces hormones that are vital for the whole body.

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Body form -

A stable but pliable substance, skin with high elasticity is plump, firm, and youthful in appearance. As we age, our skin produces and retains less elastin and collagen, leading to sagging and wrinkling of the skin. Maintaining your body form therefore requires some effort in looking after your skin.

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Minor injury -

Skin is remarkably durable, and can take a knock or two before it breaks. A light injury, a shallow cut for example, is repaired by the formation of new cells from the stratum basaleto, the deepest layer of the five layers of the epidermis. These cells repair minor injuries swiftly and effectively, without any permanent damage to the skin's surface.

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Scar tissue -

However, skin that is severely damaged may try to heal itself by forming scar tissue, which is different from normal skin tissue because it lacks hair and sweat glands.

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Temperature control -

As well as protecting the body from abrupt changes in temperature, skin also serves as a natural thermostat. It can do this because the skin's immense blood supply helps regulate temperature: dilated vessels allow for heat loss, while constricted vessels retain heat.

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Heavy skin -

Did you know that skin accounts for about 15% of your body weight? It's true!

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Dead weight -

Conversely, your skin constantly sheds dead cells, about 30,000 to 40,000 cells every minute!

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Millions of skin cells -

Here's another interesting fact: the average person has about 300 million skin cells.

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Thick skinned -

You've heard about somebody being thick skinned. But where on the body is your skin really the thickest? Well, it's on your feet, with a layer of 1.4 mm helping to cushion us as we walk and run.

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Eyelids -

At the other end of the scale, the thinnest skin on our bodies is that which covers the eyelids, only 0.2 mm in thickness. Blink and you'll miss it!

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New skin -

Skin is the one organ that's constantly changing, for better or worse. The good news is that it regenerates itself approximately every 28 days. A new coat every month, really.

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Home to bacteria -

Do you feel comfortable in your skin? Perhaps not so much after learning that it's home to more than 1,000 species of bacteria. Common skin infections include cellulitis, erysipelas, impetigo, and folliculitis.

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Skin color -

While melanin, mentioned earlier, helps protect our skin from UV rays, it's also responsible for determining our skin color.

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Dark skin -

Dark-skinned people produce more numerous and deeper-colored melanin particles, which is why people with the darkest complexions usually trace their heritage to tropical regions, where the sun is fierce and temperatures high.

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Fair skin -

Fair skin is an adaptation found in people from northern latitudes where solar rays are relatively weak. In these parts of the world, the need for bone-strengthening Vitamin D tends to be greater. But again, too much exposure to UV rays brings with it the risk of serious skin damage.

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You start aging in your twenties -

And finally, it's a sad fact of life but once you hit 25 your collagen levels can start to slowly deplete, which might result in the appearance of sagging skin and the onset of fine lines. Best advice: keep your skin moist and hydrated by drinking lots of water and staying out of strong sunshine.

Sources: (National Geographic) (Ask Nature) (National Center for Biotechnology Information) (American Academy of Family Physicians) 

See also: A guide to staying fit and healthy in middle age

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