Describe the functional importance of the heart as a pump

The heart made up of four chambers; two atria at the top and two ventricles at the bottom. There are effectively two pumps. The pump on the right side of the heart sends blood to the lungs to be oxygenated. The pump on the left side of the heart sends blood to the body to deliver the oxygen to tissues, which can use this for energy to power cells.
The typical cycle of the heart is as follows. Blood drains into the atrium on the right side of the heart. To get into ventricle, blood has to pass through an open valve. Valves are like trap doors to prevent blood moving backwards, so when enough blood has entered into the ventricle, the valve shuts. Pressure in the right ventricle builds up, heart muscle on the right side contracts, forcing open a second valve, pushing the blood to the lungs via an artery. The blood picks up oxygen in the lungs and then travels back to the left atria via a vein. The blood travels from the left atria to the left ventricle via another valve, called the mitral valve. When enough blood enters the left ventricle, the mitral valve then snaps shut, and the heart muscle contracts, causing pressure to build up in the left ventricle, and opening the aortic valve. The blood is pumped out to the body via the biggest artery in the body - the aorta, which carries blood to the tissues to deliver the oxygen.