The Human Body: An Owner's Guide

Your Amazing Machine

You own the most remarkable machine in the world, and you carry it everywhere you go. It can run, jump, heal itself, fight off germs and even grow. That machine is your body.

Most machines come with an owner's manual that explains how they work and how to look after them. This book is the owner's guide your body never came with. Let's open it up and see how the parts fit together.

Chapter 1: The Skeleton — Your Frame

Underneath your skin and muscles is a frame of bones called the skeleton. An adult skeleton has 206 bones. They do three big jobs: they give your body its shape, they let you move, and they protect your soft insides.

Your skull is like a helmet protecting your brain. Your ribs form a cage around your heart and lungs. Bones may look dry and dead, but they are very much alive — they contain blood vessels, and the spongy centre of large bones, called marrow, makes new blood cells every day.

Where two bones meet, you have a joint. Joints like your knees and elbows let you bend and move.

Chapter 2: Muscles — The Movers

Bones cannot move on their own. They need muscles, which pull on bones to create movement. You have over 600 muscles, and they make up almost half your body weight.

Muscles work by contracting, or getting shorter. Many work in pairs: one muscle pulls a bone one way, and its partner pulls it back. Try bending your arm and you can feel your biceps tighten.

Some muscles you control on purpose, like the ones in your legs. Others work automatically without you thinking about them — including the most important muscle of all, your heart.

Chapter 3: The Heart and Blood

Your heart is a muscle about the size of your fist, and it never takes a break. It beats around 100,000 times a day, pumping blood all around your body.

Blood is a delivery service. It carries oxygen and nutrients to every cell and takes away waste. Blood travels through tubes called blood vessels. If you stretched out all the blood vessels in your body, they would reach around the Earth twice.

You can feel your heart at work by finding your pulse on your wrist — each beat is your heart pushing fresh blood through your body.

Chapter 4: Lungs and Breathing

Every cell in your body needs oxygen from the air, and that is the job of your lungs. When you breathe in, air rushes into two spongy lungs in your chest.

Inside the lungs are millions of tiny air sacs where oxygen passes into your blood, and carbon dioxide (a waste gas) passes out to be breathed away. You breathe about 20,000 times a day, mostly without ever thinking about it.

Your lungs and heart are a team: the lungs collect oxygen, and the heart pumps it everywhere it is needed.

Chapter 5: Digestion — Fuel for the Body

To power all this activity, your body needs fuel, and that comes from food. Turning a sandwich into energy is the job of your digestive system.

Digestion starts in your mouth, where teeth crush food and saliva begins to break it down. The food then travels down to your stomach, which mashes it with strong acids. Next it moves into the small intestine, where the useful nutrients are absorbed into your blood. Whatever is left over passes through the large intestine and leaves your body as waste. The whole journey takes about a day.

Chapter 6: The Brain — Mission Control

In charge of everything is your brain, the body's control centre. It weighs about as much as a small melon, yet it does more than any computer.

Your brain lets you think, remember, feel emotions and learn. It also runs your body automatically — controlling your heartbeat, breathing and balance without you noticing. It communicates with the rest of the body through a network of nerves that carry electrical signals at incredible speed.

When you touch something hot and pull your hand away before you even realise it, that is your nervous system protecting you faster than you can think.

Chapter 7: Looking After Your Body

Your body is amazing, but it works best when you care for it. Three simple habits make a huge difference:

• Eat a variety of foods, especially fruit and vegetables, to give your cells the nutrients they need.
• Move every day. Exercise keeps your muscles, bones and heart strong.
• Sleep well. While you sleep, your body repairs itself and your brain stores what you learned.

You only get one body, and it will be with you for your whole life. The more you understand it, the better you can look after this extraordinary machine. To explore another world of living things, dive into Explorers of the Deep Sea.