The Skin: Our Largest Organ

The amazing coat you were born in

Stretch out your arm and look at your skin. It might not seem very exciting, but skin is one of the busiest, cleverest parts of your whole body. In fact, skin is your largest organ. An organ is a body part with an important job, and your skin is bigger than any other. If you could peel it off and lay it flat, an adult's skin would cover about 2 square metres — bigger than a beach towel.

Skin wraps around your entire body like a perfectly fitting, living coat. And unlike a coat, it can sense the world, control your temperature, keep out germs and even repair itself. Let's look at how it does all this.

The three layers of skin

Your skin is not just one thin sheet. It is made of three layers, each with its own job.

1. The epidermis is the outer layer you can see and touch. It is thin but tough, and it is waterproof. The cells on the very top are dead and flat, forming a shield. They are constantly worn away and replaced by new cells pushing up from below. The epidermis also makes a colour called melanin, which gives skin its shade and helps protect against the sun.

1. The dermis is the thicker middle layer. It is packed with useful things: tiny touch sensors, sweat glands, blood vessels, and the roots of your hairs. This is the layer that does most of the sensing and temperature work.

1. The fatty layer sits underneath. This soft, cushiony layer keeps you warm and acts like padding to protect the bones and muscles below.

A shield that keeps you safe

One of skin's most important jobs is protection. Think of it as your body's first line of defence:

• It keeps water in, so your body does not dry out.
• It keeps germs out, blocking bacteria and dirt from getting inside you. This works alongside the immune system, your body's germ-fighting team.
• It cushions and protects the softer parts inside you.

This is why a cut or graze can let germs in — the shield has a gap. Your skin quickly gets to work sealing it. A scab forms to plug the hole while new skin grows underneath. After a few days the scab falls off and fresh skin is ready. Your skin is constantly healing and renewing itself.

Keeping your temperature just right

Your body works best at a steady warmth, and your skin helps keep it there.

• When you are hot, tiny sweat glands push salty water (sweat) onto your skin. As the sweat dries, it carries heat away and cools you down. Your blood vessels also widen to let heat escape, which is why you go pink when you are hot.
• When you are cold, the blood vessels in your skin get narrower to keep heat deep inside. The tiny muscles by your hairs pull them upright, giving you goosebumps, and you might shiver to make warmth.

This constant balancing act keeps you comfortable whether you are playing in the snow or running on a hot day. It is one example of how all the systems of the human body work together.

The sense of touch

Your skin is also your sense organ for touch. Buried in the dermis are millions of tiny sensors. Some feel pressure, some feel hot and cold, and some feel pain.

These sensors keep you safe. If you touch something sharp or burning, pain sensors fire a warning to your brain in a flash, so you pull away before you are badly hurt. Your fingertips have extra sensors packed in, which is why they feel tiny details so well.

Looking after your skin

Skin does so much for you, so it is worth taking care of. Here is how:

• Wash gently to remove dirt and germs, but you do not need to scrub hard.
• Protect it from the sun. The sun gives off invisible rays called UV that can burn and damage skin. Wear sun cream, a hat and clothes on sunny days, and find shade.
• Drink water to keep your skin healthy from the inside.
• Treat cuts kindly. Clean them and let scabs do their healing work — try not to pick them.

Try it: explore your unique fingerprints

Your skin holds a secret that belongs only to you.

1. Look closely at the tip of one finger, in good light or with a magnifying glass. Can you see tiny curved lines and swirls? Those ridges are your fingerprint.
2. Rub a soft pencil onto a piece of paper to make a dark patch.
3. Press a fingertip onto the dark patch, then carefully press the same finger onto a clean piece of paper or some sticky tape. You should see your fingerprint.
4. Compare your fingerprints with a friend's or family member's.

Why it works: The ridges on your fingertips help you grip things and feel textures. Amazingly, everyone's fingerprints are different — even identical twins have their own. They form before you are born and never change, which is why police use them to tell people apart. Your skin is not just a covering; it carries a pattern that is uniquely yours.