The heart is a bag of muscle about the size and shape of a clenched fist.

Inside the heart the hollow space is divided into two halves. Each half is separated from the other by a wall of muscle which runs across the centre of the heart.

Each half is also divided into two parts by a flap called a valve. This separates each half into an upper part called an atrium and a lower part called a ventricle.

The two chambers and valve on each side of the heart act as a pump. The blood enters the upper chamber or atrium while the valve is closed. Muscles in the wall of the chamber squeeze the blood and the valve opens to let the blood through into the lower chamber. The wall of the lower chamber is made of much thicker muscle. When this squeezes the blood it gives the blood a powerful push. The valve closes to stop the blood going back into the upper chamber so the blood is forced out into an artery (main blood vessel). If the valve was weak it would let some blood go backwards and the pump would be inefficient. This would then affect the delivery of blood to the body and be damaging to health.

The two pumps in the heart work at the same time. The upper chambers take in blood together but from different places. The upper right chamber takes in oxygen-poor blood from most of the body and the left upper chamber takes in oxygen-rich blood only from the lungs. Both upper chambers squirt their blood through their valves at the same time then both lower chambers squeeze on the blood at the same time and send it out of the heart. The blood from the lower right chamber goes to the lungs and the blood from the lower left chamber goes round the body.

The beating of the heart is controlled by nerves. They work automatically so we do not have to think about making our heart beat. We can control the speed of the heart beat by our activities. When we sleep the heart beat slows down but when we are active our heart beat speeds up. The change is due to the needs of the cells. When we are sleeping we are not using our muscles so the muscle cells only need enough oxygen and food to keep them alive. When we are active our muscles need more food and oxygen to give us the energy to move.

As with all muscles, they are healthier when used hard. This is why we should do exercise or manual work and not sit about all day.