The atrioventricular (AV) valves are located between the right atrium and the right ventricle and between the left atrium and the left ventricle. The AV valve between the right atrium and the right ventricle has three cusps and is called the tricuspid valve (figure 12.7a). The AV valve between the left atrium and the left ventricle has two cusps and is called the bicuspid valve or mitral (resembling a bishop’s miter, a two-pointed hat) valve (figure 12.7b). These valves allow blood to flow from the atria into the ventricles but prevent it from flowing back into the atria. When the ventricles relax, the higher pressure in the atria forces the AV valves to open, and blood flows from the atria into the ventricles (figure 12.8a). In contrast, when the ventricles contract, blood flows toward the atria and causes the AV valves to close (figure 12.8b).
Figure 12.7 Heart Valves
Each ventricle contains cone-shaped, muscular pillars called papillary (pap′-l̄ar-ē) muscles. These muscles are attached bythin, strong, connective tissue strings called chordae tendineae (kō r′ dē ten′ di-nē -ē ; heart strings) to the free margins of the cusps of the atrioventricular valves. When the ventricles contract, the papillary muscles contract and prevent the valves from opening into the atria by pulling on the chordae tendineae attached to the valve cusps (see figures 12.6 and 12.7a; figure 12.8).
Figure 12.8 Function of the Heart Valves
The aorta and pulmonary trunk possess aortic and pulmonarysemilunar valves, respectively (see figure 12.6). Each valve consistsof three pocketlike semilunar (half-moon-shaped) cusps (see fig-ure 12.7b; figure 12.8). When the ventricles relax, the pressures in the aorta and pulmonary trunk are higher than in the ventricles. Blood flows back from the aorta or pulmonary trunk toward the ventricles and enters the pockets of the cusps, causing them to bulge toward and meet in the center of the aorta or pulmonary trunk, thus closing the vessels and blocking blood flow back into the ventricles (figure 12.8a). When the ventricles contract, the increasing pressure within the ventricles forces the semilunar valves to open (figure 12.8b).
A plate of connective tissue, sometimes called the cardiacskeleton, consists mainly of fibrous rings that surround the atrio-ventricular and semilunar valves and give them solid support (figure 12.9). This connective tissue plate also serves as electrical insulation between the atria and the ventricles and provides a rigid attachment site for cardiac muscle.