A single specialized trabeculum, the septomarginal trabecula (moderator band), forms a bridge between the lower portion of the interventricular septum and the base of the anterior papillary muscle. The septomarginal trabecula carries a portion of the cardiac conduction system, the right bundle of the atrioventricular bundle, to the anterior wall of the right ventricle.
The right atrioventricular orifice is closed during ventricular contraction by the tricuspid valve (right atrioventricular valve), so named because it usually consists of three cusps or leaflets (Fig. 3.71). The base of each cusp is secured to the fibrous ring that surrounds the atrioventricular orifice. This fibrous ring helps to maintain the shape of the opening. The cusps are continuous with each other near their bases at sites termed commissures.
The naming of the three cusps, the anterior, septal, and posterior cusps, is based on their relative position in the right ventricle. The free margins of the cusps are attached to the chordae tendineae, which arise from the tips of the papillary muscles.
During filling of the right ventricle, the tricuspid valve is open, and the three cusps project into the right ventricle.
Without the presence of a compensating mechanism, when the ventricular musculature contracts, the valve cusps would be forced upward with the flow of blood and blood would move back into the right atrium. However, contraction of the papillary muscles attached to the cusps by chordae tendineae prevents the cusps from being everted into the right atrium.
Simply put, the papillary muscles and associated chordae tendineae keep the valves closed during the dramatic changes in ventricular size that occur during contraction.
In addition, chordae tendineae from two papillary muscles attach to each cusp. This helps prevent separation of the cusps during ventricular contraction. Proper closing of the tricuspid valve causes blood to exit the right ventricle and move into the pulmonary trunk.
Necrosis of a papillary muscle following a myocardial infarction (heart attack) may result in prolapse of the related valve.
At the apex of the infundibulum, the outflow tract of the right ventricle, the opening into the pulmonary trunk is closed by the pulmonary valve (Fig. 3.71), which consists of three semilunar cusps with free edges projecting upward into the lumen of the pulmonary trunk. The free superior edge of each cusp has a middle, thickened portion, the nodule of the semilunar cusp, and a thin lateral portion, the lunula of the semilunar cusp (Fig. 3.72).
The cusps are named the left, right, and anterior semilunar cusps, relative to their fetal position before rotation of the outflow tracts from the ventricles is complete. Each cusp forms a pocket-like sinus (Fig. 3.72)—a dilation in the wall of the initial portion of the pulmonary trunk. After ventricular contraction, the recoil of blood fills these pulmonary sinuses and forces the cusps closed. This prevents blood in the pulmonary trunk from refilling the right ventricle.
The left atrium forms most of the base or posterior surface of the heart.
As with the right atrium, the left atrium is derived embryologically from two structures.
The posterior half, or inflow portion, receives the four pulmonary veins (Fig. 3.73). It has smooth walls and derives from the proximal parts of the pulmonary veins that are incorporated into the left atrium during development.
The anterior half is continuous with the left auricle. It contains musculi pectinati and derives from the embryonic primitive atrium. Unlike the crista terminalis in the right atrium, no distinct structure separates the two components of the left atrium.