The Circulatory System

The left side of the heart pumps blood around the body, the right side pumps it to the lungs. The three major types of blood vessel are arteries, veins and capillaries.

Arteries carry blood away from the heart, veins carry it back. Arteries are stronger, thicker and more flexible than veins because blood comes out of the heart at a high pressure. Veins have valves in them so that the blood can only flow one way.

The hole in the middle of a blood vessel is called the lumen. Veins have the largest lumen. The wall of capillaries are only one cell thick so that substances can pass through them.