Medical illustration showing a translucent Human Heart. The heart is about the size of its owners clenched fist and lies in the middle of the chest, where it pumps blood around the body (animation of blood circulation). It contains four chambers which are continuous with the major blood vessels. In this picture, the blue areas handle the deoxygenated blood and the red areas handle the oxygenated blood.
Deoxygenated blood (blood that has travelled around the body and lost some of its oxygen and collected some carbon dioxide) returns to the heart via the superior vena cava (blue vessel at top) and the inferior vena cava (blue vessel at bottom). From these major veins, the blood enters the right atrium (bluish chamber at middle left), which gives it a final push into the right ventricle (large bluish chamber at the front of the heart). The right ventricle contracts and pushes this blood through the pulmonary trunk and into the left and right pulmonary arteries (the blue "T" shaped vessel consists of the pulmonary trunk and arteries) and thence to the lungs where it can discharge its cargo of carbon dioxide and collect more oxygen.
Oxygenated blood returns from the lungs via the pulmonary veins (the relatively small red tubes entering the heart in the middle) and enters the left atrium. The left atrium gives a final push to drive this oxygen rich blood into the left ventricle. The left ventricle is the most powerful of the four chambers and pushes the blood out all around the body. The blood is driven through the aorta (the large red vessel that arches across the top of the heart) which repeatedly subdivides into smaller arteries.
The coronary arteries emerge from the aorta early in its course. The coronary arteries (small red vessels coursing across the surface of the heart) supply the very active heart muscle with freshly oxygenated blood. The coronary arteries are subject to blockage, leading to reduced blood flow to the heart (coronary heart disease).
The contractions of the four chambers are coordinated as the "heart beat". The two atria contract first to give a final push to fill their respective ventricles. Then the two ventricles contract together. The direction of the blood flow is maintained by a set of four valves. Two of these can be seen: in the pulmonary trunk and the aorta.
Human Heart Image (below). This image is 400 x 618 pixels; the original measures 3,000 x 4,638 pixels.