The heart is the pump that keeps blood flowing around our bodies.

The heart is made up of very strong muscle.

Inside the heart are two pumps. One pumps blood to the lungs. This is where oxygen gets into the blood. As the blood comes back from the lungs, filled with oxygen, it moves into the second pump, which pushes it around the body.

When the blood has passed around the body it has lost about a quarter of its oxygen, and it is time to become fully charged with oxygen again, so the heart pump pushes the blood back to the lungs.

The blood is pushed around the body in pulses. The heart contracts, squirting out blood and then expands again. This is why you have a pulse. Each pulse tells you that the heart has squirted another portion of blood into the arteries.

Doctors can get an idea of how well the heat is working, and whether there are problems, by measuring pulse rate and also through blood pressure.

The cuff placed around your arm in a doctor's surgery measures the pressure of the blood every time the heart works (that is called systolic pressure), and also the pressure left between beats of the heart, which is called diastolic pressure.

If you know anyone with a heart monitor, you can see all of these things working. Most children have maximum pressures of 85-120 and minimum pressures of 5-80. Blood pressure values go up as you get older.