Quick! Name the largest organ of the body!

The liver? Bzzzzz! Wrong.

The large intestine? Bzzzzz! Even wronger.

Here’s a hint: it’s waterproof, comes in a variety of designer colours, and fits any shape of body.

Yes, it’s the skin, and yes, it’s an organ, a specialized mass of tissue with a surface area of roughly 20,000 square centimetres, weighing up to five kilograms. Our skin keeps out foreign substances, retains fluids, protects us from harmful radiation, cools us and warms us, helps determine our appearance, makes Vitamin D, transmits information about our environment to our brains, and excretes water, salt and organic compounds. It’s a lot more complicated than we give it credit for!

Skin, though only between 0.4 and three millimetres deep (thinnest on the eyelids, thickest on palms and soles), has two main layers, the epidermis and the dermis, which in turn have sublayers.

The topmost layer of the epidermis, the “horny layer” (or, if you prefer your Latin straight up, the stratum corneum), consists of 25 to 30 sheets of flat dead cells that are shed and replaced constantly, at a rate of about one million every 40 minutes. (The white dust that appears in bedrooms consists mainly of dead human skin cells. Aren’t you glad I told you that?)

This constant shedding is our first line of defense against microorganisms. Every cell that flakes off takes with it whatever microorganisms may be perched on it–and even when skin is freshly soaped and rinsed clean, some areas are home to as many as three million bacteria per square centimetre.

Skin keeps the little critters at bay in other ways, too: its surface is bathed in salty sweat, which kills some microorganisms on contact, as does sebum, an oily, acidic lubricant created by glands in the dermis.

The lower layer of the epidermis contains cells called melanocytes, which produce a pigment, melanin, that determines skin colour.

Melanocytes are stimulated by exposure to ultraviolet radiation, which is harmful in excess, although some is needed for Vitamin D production. Freckles form where melanocytes are clumped together.

Every person, regardless of skin colour, has the same number of melanocytes; the only difference is in the amount of melanin produced. Dark-skinned people withstand prolonged exposure to the sun better than fair-skinned people, which is why people native to tropical and desert areas tend to have darker skin than northern people.

Beneath the epidermis is the much thicker and more complex dermis, which contains blood vessels, hair roots, glands, elastic fibers, fat and nerve endings.

The blood vessels provide nutrients to the skin cells. They’re also an important part of the body’s cooling mechanism, radiating heat into the outside world. (They also swell and allow more blood through when we are embarrassed, making us blush, an involuntary reaction unique to humans.)

Hair roots, or “follicles,” give rise to your body hair. About 100,000 of these hairs are on your scalp (unless, of course, you’ve misplaced a few of them over the years). Hair is just an outgrowth of skin (as are fingernails), made of the same stuff, keratin, and like skin, what we can see of hair is dead. Connected to each follicle is a tiny muscle whose sole purpose is to give us goose bumps.

There are two main kinds of glands in the dermis, sweat glands and sebaceous glands. The sebaceous glands, located in the follicles, excrete oily sebum through the same pore as the hair emerges.

The sweat glands produce sweat, which helps cool us. Sweat glands are also activated by certain emotional states, such as those brought on by public speaking.

The elastic fibers in the dermis and a strong, fibrous protein called collagen give skin its stretchability, strength and retractability. The fat in the skin helps cushion internal structures and also stores food energy. (Some of us have more energy in reserve than others!)

Finally, there are nerve endings that respond to touch, pressure, heat, cold and pain.

If all that sounds like a lot to cram into such a thin layer, it is. A square centimetre of skin may contain a huge number of blood vessels, 10 hairs, a dozen oil glands, 100 sweat glands, uncounted nerves and more than 200 nerve receptors.

Skin, it seems, is far more interesting than its land surface would indicate–especially if you throw on those three million bacteria per square centimetre I mentioned earlier.

It’s enough to make your skin crawl.

Permanent link to this article:

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Easy AdSense Pro by Unreal