Skin, Hair and Nails

Your skin is easy to overlook because it is always there, but it is the largest organ in your body. Spread out, an adult's skin would cover about two square metres, the size of a single bed, and it weighs around four kilograms. It keeps water in and germs out, senses the world, cools you down and even makes a vitamin.

Layers of protection

Skin has two main layers. The outer one, the epidermis, is thinner than a sheet of paper over most of the body, but much thicker on the soles of your feet. New cells are made at its base and pushed slowly upwards. As they rise they fill with a tough protein called keratin, flatten and die. The surface you can touch is a protective coat of dead, overlapping cells that are constantly flaking away and being replaced. You shed tens of thousands every minute, and much of the dust in a house is old skin. The whole outer layer renews itself roughly every four weeks.

Underneath lies the dermis, a thicker layer packed with collagen and elastic fibres that give skin its strength and stretch. The dermis holds blood vessels, nerve endings, sweat glands, oil glands and the roots of hairs. Below it, a layer of fat cushions the body and helps keep heat in.

Colour and sunlight

Skin colour comes mainly from a pigment called melanin, made by cells at the base of the epidermis. Melanin absorbs ultraviolet light from the Sun and protects the cells beneath from damage. Everyone has about the same number of pigment cells; people with darker skin make more melanin. Sunlight makes skin produce more melanin over a few days, which is what a tan is, but too much ultraviolet can burn skin and damage its DNA, raising the risk of skin cancer.

Sunlight also has a benefit. When ultraviolet rays strike the skin, they trigger the production of vitamin D, which the body needs to absorb calcium and build strong bones.

Sensing and cooling

Skin is full of sensors. Different nerve endings detect light touch, pressure, vibration, heat, cold and pain. Your fingertips and lips are especially sensitive, with tightly packed receptors. The ridges on your fingertips, which form your unique fingerprints, help you grip and improve your sense of texture.

Skin is also your cooling system. When you get hot, blood vessels in the dermis widen so more warm blood flows near the surface and gives off heat, which is why you look flushed. Millions of sweat glands release water that evaporates and carries heat away. When you are cold, the vessels narrow to save heat, and tiny muscles pull your hairs upright, giving you goosebumps.

Hair and nails

Hair and nails are both made of keratin, grown from living cells in the skin. Each hair grows from a pocket called a follicle. You have about five million follicles, most producing fine, nearly invisible hairs. Scalp hair grows about a centimetre a month, then rests and falls out, so losing around 50 to 100 hairs a day is normal.

Nails grow from a pocket of living cells at their base. They protect the tips of your fingers and toes and help you pick up small objects. Fingernails grow about twice as fast as toenails.

Healing

When you cut yourself, the skin acts fast. Blood clots seal the wound, white blood cells fight germs, and new cells spread across the gap. Deep cuts are repaired with collagen, which forms a scar. This ability to repair itself again and again is what makes skin such an effective shield.