Skin and the Integumentary System

Your skin is the body's biggest organ. Together with your hair, nails and various glands, it makes up the integumentary system (from the Latin integumentum, meaning "covering"). Skin is far more than just a wrapper. It is a tough, flexible, waterproof shield that protects you from germs, regulates your temperature, lets you sense touch, makes vitamin D from sunlight, and constantly renews itself. The average adult has about 2 square metres of skin, weighing around 4 kg: more than your liver, brain or any other single organ.

  • Skin areaapprox. 2 m²In an average adult
  • Skin weightapprox. 4 kgAbout 7% of body weight
  • Three layersEpidermis, dermis, hypodermisOuter to inner
  • Skin cells shed per dayapprox. 30,000 to 40,000approx. 9 lbs per year
  • Hairs on the bodyapprox. 5 millionapprox. 100,000 on the head
  • Sweat glandsapprox. 3 millionMostly on hands, feet and forehead

The three layers of skin

  • Epidermis: the thin outer layer (just 0.1 mm thick on most of the body). The top is made of dead, flattened cells that are constantly being shed. New cells are produced underneath and gradually migrate to the surface.
  • Dermis: the thicker middle layer (1 to 2 mm). Contains hair follicles, sweat glands, blood vessels, nerve endings and tough collagen fibres that give skin its strength and flexibility.
  • Hypodermis (also called the subcutaneous layer): the deepest layer, made mostly of fat. Insulates the body, stores energy, and connects the skin to the muscles below.

What skin does for you

The skin is doing many jobs at once.

  • Barrier: keeps germs, dirt and water out, and keeps water in. Without skin, you would dry out within hours.
  • Temperature control: blood vessels in the dermis expand to lose heat or constrict to keep heat in. Sweat glands cool you by releasing water that evaporates.
  • Touch sense: millions of nerve endings let you feel pressure, vibration, pain and temperature.
  • Vitamin D production: when sunlight (UV-B) hits your skin, it triggers production of vitamin D, which your body needs for healthy bones.
  • Defence against UV: special cells called melanocytes produce melanin, a pigment that absorbs UV light and prevents it damaging deeper layers.
  • Immune defence: special skin immune cells spot any germs that get through the outer barrier.
  • Waste removal: small amounts of waste (salt, urea, lactic acid) are removed in sweat.

Skin is constantly renewing

The top layer of your skin is mostly dead cells that are constantly being shed and replaced. You lose around 30,000 to 40,000 dead skin cells every minute: more than 4 kg of dead skin per year. Most household dust is actually flakes of human skin.

New skin cells are made deep in the epidermis. They migrate up over about 4 weeks, gradually filling with a tough protein called keratin and dying as they reach the surface. By the time they shed, they are part of the protective outer layer for one last brief moment.

Hair and nails

Both hair and nails are made of keratin: a tough protein. Both are technically dead by the time they emerge: only the very root (still inside the skin) is alive, and that is where growth happens.

You have about 5 million hairs on your body, including roughly 100,000 on your head. Different hairs have different functions:

  • Eyebrows and eyelashes protect your eyes from sweat and debris.
  • Nostril and ear hairs filter out particles.
  • Head hair provides insulation and (probably) protection from sunburn.
  • Body hair was once important for warmth in our distant evolutionary past; today it is mostly vestigial.

Fingernails grow at about 3 mm per month; toenails grow at about 1.5 mm per month. Nails protect the sensitive tips of your fingers and toes, and help with grip and fine manipulation.

Sweating: built-in air conditioning

Sweat is your body's way of cooling itself. You have around 3 million sweat glands scattered across your skin, concentrated on the palms of your hands, soles of your feet, armpits and forehead. When your body gets too hot, these glands release watery sweat onto your skin. As the sweat evaporates, it cools the skin (because evaporation absorbs heat).

A person doing heavy exercise in hot weather can produce 2 to 4 litres of sweat per hour. This is one of the main reasons humans are some of the best long-distance runners on Earth: very few animals can lose heat as efficiently as we can.

Fact Your skin colour comes from the same pigment that gives a banana its yellow streaks: melanin. Cells in your skin called melanocytes produce melanin, which absorbs harmful UV light from the Sun. People with naturally dark skin produce more melanin and have evolved that way over many generations in sunnier climates; people with naturally light skin produce less and are better adapted to weaker sunlight. Tanning is your skin's way of making more melanin to protect itself when it gets a lot of sun.

Why skin is so good at healing

When you cut yourself, your skin springs into action immediately.

  1. Blood vessels at the wound contract to reduce bleeding.
  2. Platelets stick to the wound and form a clot.
  3. White blood cells move in to fight any germs that have entered.
  4. New skin cells start dividing under the scab and rebuilding the damaged tissue.
  5. Over several weeks, the wound closes and the scab eventually falls off.

Most small cuts heal completely with no scar. Deeper cuts may leave a scar, where new tissue forms a slightly different pattern from the original skin.

Did you know? Your skin is unique to you, just like your DNA. Your fingerprints form in the womb before you are even born and stay essentially the same for your whole life. No two people, not even identical twins, have exactly the same fingerprints. This is why fingerprints are still one of the most reliable ways to identify people more than a century after they were first used by police.
Deeper dive: why your skin can do photochemistry that no machine can

One of the most surprising things your skin does is make vitamin D from sunlight. Vitamin D is essential for your bones (it helps you absorb calcium from food), your immune system and many other body processes. Unlike other vitamins, you do not have to eat it: your skin produces it directly when sunlight hits it.

Here is how it works. Deep in your skin sits a chemical called 7-dehydrocholesterol. When UV-B radiation from the Sun (wavelength around 290 to 315 nm) hits this chemical, it triggers a precise photochemical reaction that converts it into previtamin D3. Body heat then rearranges the molecule into vitamin D3, which travels in the blood to the liver and kidneys to be activated into its final useful form.

This is one of the few biological reactions in the human body that is directly driven by light. It is also why people who live at high latitudes (like Britain, especially in winter) are often vitamin D deficient: the Sun does not get high enough in the sky to deliver enough UV-B. Eating vitamin D-rich foods (oily fish, eggs, fortified milk) or taking supplements is often recommended in winter.

Skin colour is closely tied to this vitamin D balancing act. People with darker skin have more melanin, which blocks UV light and protects against skin damage but also reduces vitamin D production. In sunny equatorial climates this is a good trade. In cloudier high-latitude climates, lighter skin evolved to let in more UV and make enough vitamin D. The geographical pattern of human skin colour around the world fits this trade-off remarkably well, which is one of the clearer examples of recent human evolution.

For the immune system that defends what skin lets through, see the immune system. For the underlying framework of bones, see the skeletal system.