The biggest blood vessels are called arteries. Arteries have thick walls made of muscle and fibres. The fibres add strength to the walls but also are elastic to allow the arteries to stretch. Blood is pumped, by the heart, into the arteries at high pressure which causes the artery walls to expand. The muscles and the elastic fibres then push back inwards to force the blood away from the heart. The arteries are therefore expanding and contracting to keep the blood moving, it is this expansion and contraction that you can feel as your pulse. Capillaries are very small tubes. These branch out in all directions from the arteries and carry the blood to all the different parts of the body. When you scratch yourself or graze your knee the blood that comes out is usually from capillaries because they are close to the skin. Veins are smaller than arteries and have thinner walls. By the time the blood reaches the veins it is at a lower pressure. Veins take the blood back to the heart and it is important that the blood travels in the right direction. This is done in two ways. Inside the veins are many small valves which allow the blood through in one direction but stop it flowing backwards. These valves are like small flaps. Secondly, veins are positioned inside some of the larger muscles of the body and as you move these muscles squash the veins and then release them to keep the blood moving.