The heart pushes on the blood and makes it move through pipes called blood vessels. There are two sets of blood vessels. One takes the blood from the heart to the lungs and back to the heart. The other takes the blood from the heart to all the other organs of the body and then back to the heart. This arrangement of two sets of blood vessels allows the cells to be served efficiently. When the blood has picked up oxygen in the lungs it returns to the heart to be given an extra push to send it all the way round the body. If there was just one set of blood vessels the heart would not be able to give this vital extra push and the blood would travel too slowly to keep the cells alive.

There are three kinds of blood vessels. Each one is adapted for a particular purpose. The blood vessels which take blood away from the heart are called arteries. The heart pushes the blood out with great force in one beat then rests a moment and sends out more in the next beat. This means that the artery is receiving a strong push from the blood then there is a pause then there is a strong push again. The artery has a thick wall to stand up to the pushing or pressure of the blood. The wall is also elastic so that it can stretch with the high pressure and shrink back again when the pressure is lower between heart beats.

When the blood leaves the arteries it enters tiny blood vessels called capillaries. They form networks through every organ of the body so that no cell is more than 0.1 mm away from a capillary and its supply of blood. As the blood enters a network of capillaries its pushing force or pressure falls very low so the capillary walls do not need to be thick. In fact they are very thin. They are made of just one layer of cells. The thinnest of the capillary walls lets oxygen and food pass quickly from the blood to the surrounding cells and lets wastes pass quickly from the cells to the blood. If the capillary walls were thick, materials would not be able to move quickly through them and the cells around the capillary would die.

The blood enters the veins after leaving the capillaries. The pressure of the blood increases again but it is not as strong as in the arteries. There is a danger of the blood flowing backwards which would make the circulation inefficient so the veins are equipped with valves. The vein walls are thinner than artery walls because they do not need to stand up to the same high pressure. The thinner walls also helps the vein to move the blood forwards. When you exercise, the muscles around a vein squeeze it because it has thin walls. The squeezing of the vein also squeezes the blood and pushes it through the valves towards the heart.