Blood is a mixture of different things. Most grown-ups have between four and six litres of the stuff moving around their body all the time. The mixture is made up of:

• red blood cells which carry oxygen throughout the body
• white blood cells which fight infections
• platelets which are cells that help you stop bleeding if you get a cut
• plasma a yellowish liquid (some of which is water) that carries nutrients (the best bits of food), hormones and proteins throughout the body

There are two colours of blood in the body:

• Red blood – filled with oxygen ready to travel to all parts of the body
• Blue blood – with no oxygen in it that is ready to travel back to the lungs where it will pick up a new supply of oxygen

Blood moves around the body in blood vessels, arteries and veins. A very fancy transport system, like roads, delivering important things to every part of the body.

The blood can only travel around the body because of the heart because it is the heart that pumps it round, so why does blood need to travel around the body? Because every part of the body need to refuel with lots of fresh new stocks of good things like oxygen, protein and nutrients to make energy and grow. Blood also carries cells that fight off infection and that repair us when we injure ourselves or bleed.

Making energy: the mixture of oxygen, nutrients and water in the blood help every small part of the body make energy. That energy helps the body grow, helps the brain think, helps the arms and legs move, the eyes see, the ears hear, helps you eat food and it even helps you watch TV, kick a football, ride a bike or play computer games!

After the body has used up all the goodness it wants to get rid of anything it no longer needs, the rubbish, so it sends it all back in the blood.

The heart has two halves.

The left side of the heart collects oxygen filled blood from the lungs in its top chamber, the atrium. It is a bright red colour and is filled with all of the ingredients needed to make energy. It then travels into the lower pumping section of the heart, the ventricle, which has a strong muscle that pushes blood out of the heart into the biggest blood vessel in the body, the Aorta, that we call the motorway of the body. Leading off the Aorta there are lots of big blood vessels called arteries that take blood off to the brain, kidneys, liver, gut and eventually down the legs and arms.

The body takes what it wants from the blood so that it can make energy leaving the blood empty of oxygen and turning the blood a blue colour .

This blue blood needs to travel back to the lungs which is where it will pick up a new supply of oxygen so lots of blood vessels called veins carry the blood back to the right side of the heart. Where it is collected in the collecting chamber, atrium, then is pushed into the pumping chamber, ventricle which pushes it up to the lungs via the pulmonary artery.

Simple rule – Arteries carry red, oxygen filled blood away from the heart to the body. Veins carry blue, oxygen empty blood back to the heart and lungs.

Of course the heart does not look like the box picture above it looks more like this.

All of the different parts of the heart are the same as described above but you can see the different section/chambers of the heart. Follow the arrows on the same journey as the described above starting in the Left Atrium, collecting chamber, down through the mitral valve into the left ventricle or pumping chamber then up through the aortic valve into the aorta, the motorway artery of the body. All of this part of the heart is filled with red oxygen filled blood.

Once the body has used up all of the oxygen it turns blue and then returns to the heart on the right side of the heart. Follow the arrows from the head and the body as blood arrives in the right atrium, collecting chamber, down through the tricuspid valve into the right ventricle, pumping chamber, which pushes the blood up through the pulmonary valve into the pulmonary artery into the lungs.

The lungs do their job and take loads of waste gases, carbon dioxide, out of the blood and then fill the blood up with oxygen. The blood then starts the journey all over again.

To learn more about every part of the heart visit the Normal Heart page of the main website – click here.

You can also visit Kids Health which shows a great cartoon of how the heart works – click here.

So the heart pumps blood 60 to 80 times each minute, up to 120 times if you are a baby. Each heart pump is a heart beat. Sometime you can feel you heart beating if you put your hand on the left side of your chest. You can also feel you heart beat as a pulse in your wrist.

The heart is very clever it pumps all by itself, in two parts. The first part of the pump is when the collecting chambers empty into the ventricles and then the second part of the pump is when the ventricles empty into either the pulmonary artery or the aorta.

The heart has its own little electrical computers to tell the two bits of the heart to pump. The beat starts in the sino atrial node in the top part of the heart, travels through the collecting chamber, atrium down to the atrio ventricular node in the middle of the heart that then sends the beat through the two pumping chamber, ventricles. Clever!

When you have an ECG the beat is recorded like this.

The letters each show a different part of the one beat.

Next time you have an ECG ask the doctor to point out your heart rhythm.