Gross Anatomy of the Heart (Figure 2a)

The heart consists of four chambers. Blood flows into the right atrium via the superior and inferior venae cavae. The left and right atria connect to the ventricles via the mitral (two cusps) and tricuspid (three cusps) atrioventricular (AV) valves, respectively. The AV valves are passive and close when the ventricular pressure exceeds that in the atrium. They are prevented from being everted into the atria during systole by fine cords (chordae tendineae) attached between the free margins of the cusps and the papillary muscles, which contract during systole. The outflow from the right ventricle passes through the pulmonary semilunar valve to the pulmonary artery, and that from the left ventricle enters the aorta via the aortic semilunar valve. These valves close passively at the end of systole, when ventricular pressure falls below that of the arteries. Both semilunar valves have three cusps.

The cusps or leaflets of the cardiac valves are formed of fibrous connective tissue, covered in a thin layer of cells similar to and contiguous with the endocardium (AV valves and ventricular surface of semilunar valves) and endothelium (vascular side of semilunar valves). When closed, the cusps form a tight seal (come to apposition) at the commissures (line at which the edges of the leaflets meet).

The atria and ventricles are separated by a band of fibrous connective tissue called the annulus fibrosus, which provides a skeleton for attachment of the muscle and insertion of the valves. It also prevents electrical conduction between the atria and ventricles except at the atrioventricular node (AVN). This is situated near the interatrial septum and the mouth of the coronary sinus and is an important element of the cardiac electrical conduction system (see Chapter 13).

The ventricles fill during diastole; at the initiation of the heart beat the atria contract and complete ventricular filling. As the ventricles contract the pressure rises sharply, closing the AV valves. When ventricular pressure exceeds the pulmonary artery or aortic pressure, the semilunar valves open and ejection occurs (see Chapter 16). As systole ends and ventricular pressure falls, the semilunar valves are closed by backflow of blood from the arteries.

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