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How Your Heart Works

The heart is a hollow, muscular organ located behind the breastbone (sternum) and between the lungs in the middle of the chest. The normal adult heart is about the size of two clenched fists and, depending upon your size and weight, it weighs 7 to 15 ounces.

The heart is divided into four chambers. After blood circulates through the body, it is returned to the heart through a network of veins that culminate in the superior vena cava, which empties the blood into the heart's upper right atrium. It is then forced through the tricuspid valve into the right ventricle. During a heartbeat, the right ventricle contracts to force blood through the pulmonary valve and into the lungs. There carbon dioxide and other waste gases are exchanged for a fresh load of oxygen and the oxygenated blood is returned to the heart's left atrium. It passes through the mitral valve into the left ventricle, the heart's main pumping chamber. With each heartbeat, a few ounces of fresh blood are forced through the aortic valve into the aorta - the body's largest artery - to begin a new journey through the body's blood vessels.

The Heart's Electrical System

The heart's rhythmic beating action is controlled by coordinated electrical impulses. Normally, the electrical impulses originate in the heart's natural pacemaker, the sinus node. This tiny bundle of specialized cells is located in the top right corner of the heart. The electrical impulses travel through the muscle fibers of the two atria (the heart's upper chambers), to a bundle of cells called the atrioventricular node, which is located in the juncture between the right and left sides of the heart where the right atrium and right ventricle meet. The impulses then travel along two fibrous pathways called the bundle of His and the Purkinje fibers, which are named for the scientists who first described them. The heartbeat occurs when the electrical impulses stimulate the muscle cells (the myocardium) to contract, which forces blood through the one-way valves that connect the various heart chambers.