The heart is divided into four chambers: the right atrium and right ventricle, and the left atrium and left ventricle. The walls of the chambers are made of a special muscle, the myocardium, which contracts rhythmically under the stimulation of electrical currents.

A wall of muscle called the septum (atrial septum for the atria and ventricular septum for the ventricles) separates the left and right atria and the left and right ventricles from each other.

Blood in the heart is kept flowing in a forward direction by a system of four one-way valves, each closing off one of the heart’s chambers at the appropriate time in the cardiac cycle. The valves open to let the blood through when the chambers contract, and snap shut to prevent it from flowing backwards (regurgitation) as the chambers relax. The valve system also helps maintain different pressures on the right and left sides of the heart.

The two valves separating the ventricles from the circulatory system are called semilunar because of their crescent-shaped cusps. At the juncture of the right ventricle and the pulmonary artery lies the pulmonary valve. The other semilunar valve, the aortic valve, lies between the left ventricle and the aorta.

The valve located between the left ventricle and left atrium is a cone-shaped funnel called the mitral valve. The corresponding valve between the right ventricle and right atrium is called the tricuspid valve. As its name suggests, it has three cusps, or leaflets, which are thinner than those of the mitral valve.