How does the skin work?

Your skin is an organ, and the biggest one you have: about 1.5 to 2 m², the size of a single bed sheet, and about 3.5 to 4 kg. It keeps water in and germs out, holds your temperature at 37 °C, feels the world, and even makes vitamin D.

This is a tiny block of it, magnified about 20 times, cut open like a model. On top is the epidermis, only about 0.1 mm thick on most of the body (thicker on your soles, thinner on your eyelids; we drew it thicker so you can see it). Its outer skin, the stratum corneum, is made of dead, flattened cells. At its base, the basal layer makes new cells, and melanocytes there make the pigment melanin.

Under it is the dermis, about 1 to 4 mm of tough, stretchy tissue made of collagen and elastin fibres. It holds the blood vessels, nerves and touch receptors, the hair follicles with their sebaceous (oil) glands and tiny arrector pili muscles, and the coiled sweat glands. Its bumpy top, the dermal papillae, locks into the epidermis like a zip so the layers don’t slide apart. Deepest is the hypodermis, a cushion of fat that stores energy and keeps heat in.

The outer skin you touch is dead. It is made in the basal layer at the bottom of the epidermis, where cells called keratinocytes divide. Each new cell is pushed upwards by the ones born under it.

As it rises, it changes. In the spinous layer it fills with tough keratin fibres. In the granular layer it makes waxy fats and loses its nucleus. By the top it has died and flattened into a corneocyte, a thin plate of keratin. Thousands of these plates stack like bricks, glued by fatty mortar: the stratum corneum. After about 4 weeks from birth (longer as we age) a cell flakes off.

This brick wall is the barrier. It stops you drying out: only about 300 to 400 mL of water a day seeps through healthy skin. Lose it, as after a big burn, and water loss can rise about tenfold. It also keeps most germs out. Water moving across layers like this is osmosis and diffusion, the subject of a future OsmosisClear box.

You shed roughly 500 million skin cells a day. They are part of house dust, but the popular claim that dust is mostly skin is exaggerated: much of it is soil, fibres and other bits.

Your body makes heat all the time, about 100 watts at rest, like a bright old light bulb, and several times more when you run. To stay near 37 °C inside, it has to lose exactly as much heat as it makes, and the skin is where it does it.

Blood vessels are the first dial. When you are warm, vessels in the dermis widen and send warm blood to the surface to lose heat: you look flushed. In the cold they narrow and keep blood deep: fingers go pale and cold. Skin blood flow can go from almost nothing to about 7 litres a minute.

Sweat is the big one. Turning 1 gram of sweat into vapour takes about 2,400 joules of heat, pulled from your skin. You can sweat 1 to 2 litres an hour. But sweat only cools when it evaporates. In humid air it can’t dry, drips off and cools you very little. A breeze helps by carrying damp air away: see the cooling chapter of FanClear. Heat always flows from hot to cold, as ThermoClear shows (a future HeatClear box will dig into heat transfer).

Scientists combine heat and humidity into the wet-bulb temperature. Above about 35 °C wet-bulb, even a healthy person resting in the shade can’t cool down, and lab tests suggest the real limit is lower, about 31 °C. Humid heat waves in India make this very real, so rest, shade and water matter. Goosebumps are the arrector pili standing hairs up: in furry animals that traps warm air; in us it is a leftover.

Your skin feels with receptors: nerve endings, some wrapped in special capsules, each tuned to a different kind of touch. This is fingertip skin: no hair, friction ridges (your fingerprint) and some of the most crowded receptors in the body.

Merkel cells sit at the base of the epidermis and report steady pressure, edges and shapes; they keep firing while you press. Meissner corpuscles, tucked into the dermal papillae, feel light touch and slipping, and fire as a touch starts and stops, best at a flutter of about 30 Hz. Deep down, onion-like Pacinian corpuscles feel vibration, most sharply at about 250 Hz, like a phone buzzing. Ruffini endings feel the skin stretch. Bare free nerve endings reach right into the epidermis and report pain, heat, cold and itch.

How finely you can feel depends on how close together the receptors and their patches of skin are. In the two-point test a doctor touches you with two points: on a fingertip you can tell them apart at about 2 to 3 mm; on your back they must be about 40 mm apart. The brain gives sensitive parts more space in its touch map, the famous “homunculus” you can explore in BrainClear.

Every person has about the same number of melanocytes. Skin colour comes from how much melanin they make and which kind: brown-black eumelanin or red-yellow pheomelanin. Melanocytes pack it into tiny grains and pass them to nearby skin cells, where it sits over the nucleus like a little umbrella.

Melanin absorbs ultraviolet (UV) light, which is just beyond violet on the spectrum: more energetic than visible light, far less than the X-rays in XrayClear. UV-B mostly stops in the epidermis, causes sunburn and damages DNA. UV-A goes deeper into the dermis and ages skin. Light skin lets roughly five times more UV-B into the dermis than deeply pigmented skin. Sun exposure makes melanocytes produce more melanin: a tan, which protects only a little. UV-B is also useful: it makes vitamin D in the epidermis.

The UV index tells you how strong the sunburning UV is. Around summer noon, Indian cities often reach 10 to 12 or more, “very high” to “extreme”. SPF 30 sunscreen, used thickly as in the lab, blocks about 97% of sunburning UV (1 in 30 gets through). Everyone can burn, and every skin tone can get skin cancer, so for your own skin, ask a doctor or dermatologist.

Why so many shades? Our ancestors lived under Africa’s strong sun, where dark skin protects folate, a vitamin vital for healthy babies. As people moved to places with weaker sun, lighter skin let enough UV-B in to make vitamin D (Nina Jablonski and George Chaplin, 2000). India, with its sunny south, high mountains and long history of people mixing, has nearly the whole range. Skin colour is an adaptation to sunlight. It says nothing about anyone’s worth. Doctors also use the Fitzpatrick scale (1975), which sorts skin by how it reacts to sun, not by colour.

Skin gets hurt more than any other organ, and it is very good at mending. This chapter shares general facts only: for anything about your own skin, see a doctor or dermatologist.

A cut heals in overlapping stages. Within minutes, platelets and a fibrin mesh form a clot. For a few days white blood cells clean the wound, so the edges look red. Then pink granulation tissue fills the gap and new skin cells crawl across under the scab. For months after, collagen is rebuilt. The result is a scar: it has no hair, sweat glands or ridges, and at best about 80% of the original strength.

Burns are graded by depth. First degree reaches only the epidermis (like mild sunburn). Second degree reaches the dermis and blisters. Third degree destroys the whole skin, even its nerve endings, and usually needs skin grafts. The WHO counts over a million serious burns a year in India, many from kitchen fires and hot liquids; first-aid guides say to cool a burn under clean, cool running water and to get medical help for anything more than a small burn.

Acne starts when sebum and dead cells plug a follicle; bacteria grow and the body reacts with a red spot. Most teenagers get some. Eczema is partly a leaky barrier: water escapes and irritants get in, so skin turns dry and itchy. Moles are clusters of melanocytes. Most are harmless, but the ABCDEs help spot ones to show a dermatologist. In darker skin, melanoma is rarer but more often appears on palms, soles or under nails.