The right side of the heart receives blood from the veins in the body and pumps it through the pulmonary artery to the lungs. There, it picks up fresh oxygen and releases carbon dioxide and then passes through to the left side of the heart.
The left side of the heart receives oxygen-rich blood from the arteries in the lungs, and pumps it through the aorta to the body.
The illustration below shows the heart, the arrows show the direction the blood flows in and the colour shows the oxygen levels of the blood, red is blood carrying oxygen and blue is blood with no or low oxygen.
With each heartbeat, the heart pumps blood forward from the left side of the heart through the aorta and into the arteries. The arteries divide off into smaller and smaller branches to supply a microscopic network of capillaries, taking the blood to every part of your body, even to the heart itself.
The blood travels back to the heart from all areas of the body having given the nutrients and oxygen required. It starts off at the capillaries, joins to veins and then much larger veins and back to the right side of the heart (blue on the diagram).
Between each heart beat the heart relaxes and blood fills its right side. When it contracts again this blood is forced to the lungs to retrieve oxygen and then continues to the left side of the heart. The two sides of the heart are separate but they work together. The right side of the heart receives dark, de-oxygenated blood which has circulated around your body. It pumps this to your lungs where, upon picking up a fresh supply of oxygen, it becomes bright red.
Each side of the heart has a thin-walled ‘collecting chamber’ (the atrium) which helps to fill the thick-walled main pump (the ventricle). The walls of the heart are made of specialised muscle cells called myocardium. Like every other living tissue, the myocardium itself needs a continuous supply of fresh blood. This supply of blood comes from the coronary arteries as it leaves the left ventricle. The coronary arteries spread across the outside of the myocardium, feeding it with a supply of blood.
The system explained above is called the circulatory system and serves to pump the eight pints (or five litres) of blood contained in your body from head to toe, recirculating the blood constantly. Each day, your heart beats about 100,000 times and pumps about 23,000 litres (5,000 gallons) of blood through it.