The heart is the hollow, muscular organ in the thoracic cavity (chest) that maintains the circulation of blood throughout the body. It is surrounded by a membrane called the pericardium. The pericardium consists of a layer of fibrous connective tissue and a layer of thin, serous (i.e., produces a secretion) tissue and is attached to the vena cava, the aorta, the diaphragm, and the sternum. The pericardial cavity, the potential space between the pericardium and the heart, contains the watery pericardial fluid. This fluid prevents friction between the pericardium and the heart.

The heart wall consists of the epicardium (inner layer), the myocardium (middle layer comprised of cardiac muscle tissue), and the endocardium (lining of the myocardium that covers the heart valves).

The heart has a right side and a left side. Each side has a relatively thin-walled chamber that receives blood returning to the heart (atrium) and a muscular chamber that pumps blood out of the heart (ventricle).

Blood Flow
The flow of blood through the heart is controlled by the opening and closing of valves and the contraction and relaxation of the myocardium. Heart valves are controlled by pressure changes within each chamber and contraction and relaxation are controlled by the heart’s conduction system.

Blood that has traveled through the body returns to the heart and enters the right atrium. This blood flows through the tricuspid valve into the right ventricle. The right ventricle pumps the blood to the lungs, where it absorbs oxygen. Oxygen-rich blood returns from the lungs and enters the heart through the left atrium. Blood passes from the left atrium through the mitral valve and into the left ventricle.

The left ventricle, the largest and most muscular of the four chambers, is the main pumping chamber of the heart. When the left ventricle contracts, blood is pumped through the aortic valve into the main artery of the body (aorta). The aorta supplies blood to smaller arteries that travel to the head, arms, abdomen, and legs. These arteries supply oxygen-rich blood to the organs and tissues of the body, which require oxygen to function. The coronary arteries supply oxygen-rich blood to the tissues of the heart.

Oxygen-poor blood travels from organs and tissues to the heart through veins. The vena cava is the major vein that returns blood to the right atrium of the heart. The vena cava superior returns blood from the head, neck, upper extremities, and chest. The vena cava inferior returns blood from the lower extremities, the pelvis, and the abdomen. The coronary sinus drains blood from the coronary arteries into the right atrium.

Conduction System
An electrical impulse travels through the heart and initiates contractions of the chambers. The heart’s “spark plug” is an area of specialized heart tissue called the sinoatrial node (SA node), which is located in the right atrium. Each time the SA node “fires,” an electrical impulse is generated that travels through the right and left atria, signaling these chambers to contract and pump blood into the ventricles.

The impulse then travels into another area of specialized heart tissue called the atrioventricular node (AV node), which is located between the atria and the ventricles. The electrical impulse is conducted through the AV node and wire-like pathways (Purkinje fibers) to the ventricles, signaling the ventricles to contract and pump blood into the lungs and throughout the body.

The normal sequence of electrical activation of the chambers of the heart is called sinus rhythm. It occurs each time the heart beats, usually about 60 to 80 times every minute. In a normal heartbeat, the atria contract simultaneously while the ventricles relax. Then, the ventricles relax and the atria contract. The term systole refers to contraction and the term diastole refers to relaxation. A heartbeat consists of the systole and diastole of the atria and the systole and diastole of the ventricles.