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• There are three types of blood vessel: arteries, veins and capillaries.
• The heart is a muscular organ that pumps blood around your circulatory system.
• The circulatory system is the heart and all the blood vessels in the body which carry cells and substances to all its parts.

There are three types of blood vessel:

• Arteries carry blood away from the heart straight after it has been pumped. This means it is under high pressure. The walls of arteries are made of thick muscle to withstand this pressure. This muscle is also elastic to allow a pulse of blood to travel along when your heart beats.

• Veins carry blood back to the heart. This blood is under lower pressure because some of it has been lost as it travelled around your body. The walls of veins are made of thinner muscle and are less elastic than arteries. Because the blood is under lower pressure, one-way valves in your veins stop it flowing backwards.

• Capillaries are the tiny blood vessels that branch out into every tissue in your body carrying substances your cells need like oxygen and glucose for respiration and for removing waste products like carbon dioxide. They have very thin walls to allow these substances to move by diffusion in and out of your cells. Capillaries join your arteries to your veins.

The heart is a large muscular organ which pumps blood to the lungs and then the rest of your body. It has four chambers.

The two top chambers are called atria (right atrium and left atrium) and the bottom chambers are called ventricles (right ventricle and left ventricle). The atria collect blood and then pump them to the ventricles below. The ventricles then pump the blood to the body. It looks as though the ventricles are bigger but the four chambers inside are the same size. It is the muscular lining of the ventricles that are bigger because they have to pump the blood further than the atria.

Humans have a double circulatory system, which means the blood passes through the heart twice on each loop around the body. It travels from the heart to the lungs, to the heart, to the rest of the body before returning to the heart. This is often called a 'figure of eight system'.