The heart is a pumping organ that keeps the blood flowing around the body.
The walls of the heart are mostley made up of muscle tissue.
The heart has valves to make sure that blood goes in the right direction - they prevent it flowing backwards.
How the blood uses its four chambers
1) Blood flows into the two atria from the vena cava and the pulmonary vein.
2) The atria contract, pushing the blood into the ventricles.
3) The ventricles contract, forcing the blood into the pulmonary artery and the aorta, and out of the heart.
4) The blood then flows to the organs through arteries, and returns through veins.
5) The atria fill again and the whole cycle starts over.