Can you imagine a coat tailored from a sole piece, covering the body from head to feet, protecting against wind, being water-proof, strong but still elastic, and continuously renewing itself? Well, this is your skin!
1. The skin is our largest organ. Extended, it has a surface of about 2 square meters (18 sq ft), about the size of a normal sheet. It weighs on average 3 kg (8.5 pounds), about 5 % of our body weight. Its thickness varies between 0.5 to 5 cm (0.2 to 2 in), being thinner on the body parts unexposed to environment and pressure, and thicker on the most solicited parts, like the sole.
2. The skin is made of two layers. The external horny layer is called epidermis, being made of 20-30 rows of dead cells, located one above the other like tile on a roof (that's why skin stretches easily during our movements). Daily, thousands of cells are shed from the skin under the form of tiny scales (on the head, sometimes as visible dandruff), but this does not erode because the lost cells are continuously replaced by new ones, produced by the basal germinative layer.
The new cells are loaded with keratin, a hardy protein, extremely resistant to variations of temperature and humidity. The same protein is the main component of the nails, claws, hooves, feathers, hairs, horns (including the rhino's). A new cells arrives to the external layer in 3-4 weeks (its lifetime). We lose this way about 18 kg (40 pounds) of skin in a life time.
If the epidermis gets cut or bruised, the skin heals without leaving any scar.
3. Under the epidermis, the pigment cells (melanocytes) are located. They produce the melanin pigment that protects us against UV rays. When exposed to sunlight, melanocytes produce more melanin, making the skin darker: we get tanned. Later, the melanocytes migrate to the upper skin and are shed with the keratin cells, that's why the tan is lost.
Darker skin means more melanin in the melanocytes. People with lighter skin get easier sun burns, due to their lower melanin amount in the skin.
4. The second layer of the skin is called derm. Its upper portion contains collagen and elastin, proteins that confer skin's elasticity and flexibility. When getting older, these protein fibers degrades. This, combined with lower sebum production, induces the wrinkles.
The derm contains numerous blood vessels that dilate, filling with blood when we get warm. This way we lose more heat, and the body cools down. This is why we blush when it's hot.
If it's cold, the derm's vessels become narrower losing less heat, and the skin turns paler. If the skin is traumatized, the blood clots and closes the wound, protecting against germs and further blood loss.
5. The derm is filled of nerve terminations. These terminations detect touch (Meissner corpuscles for fine touch, Vater-Puccini for tough pressure), pain, temperature (Ruffini corpuscles for heat, Krause corpuscles for cold). The simple terminations for pain sensation penetrate into the epidermis.
The tip of the fingers represent the body's richest portions in skin sensors.
6. Deep in the derm are located the sweat glands. Sweat has the role of cooling down the skin and eliminate toxins (we sweat even in cold days!) and salt excess (that's why sweat is salty). We produce on average 250-500 ml of sweat daily and this can rise to 2 l during the hot days. A human being has about 3 million sweat glands, weighing about 100 g (4 onces). They are concentrated on the face, armpit, sole and palm. There are about 350 sweat glands per square centimeter on the face of the palm, and just 200 per square centimeter on the back of the palm.
Sweat also eases the grabbing smooth objects. It is believed that this is the main reason why primates kept sweat glands why many mammals lost them: they help climbing the trees.
7. The derm also harbors the hair follicles (the roots, the living part of the derm). The sebaceous glands from the derm produce the sebum, the greasy stuff that lubricate the skin and the hair. Without the sebum, the skin would turn quickly dry and get scaly, lacking resistance. Each hair has a muscle fiber located in the derm.
When scared or getting cold, the muscles contract, raising the hairs, forming the chicken skin.
8. Under the derm, there is the fatty tissue, functioning as isolator for the body against variations of temperature: it impedes heat loss, protecting against cold. When necessary, the body uses the fatty tissues as fuel source. All the food excesses are deposited here.
9. In the sunlight, vitamin D is formed in the derm.
10. The skin protects the body against blows, cuts, rain, wind, radiations, powerful sunlight, and germs.