The space posterior to the crista is the sinus of venae cavae and is derived embryologically from the right horn of the sinus venosus. This component of the right atrium has smooth, thin walls, and both venae cavae empty into this space.
The space anterior to the crista, including the right auricle, is sometimes referred to as the atrium proper. This terminology is based on its origin from the embryonic primitive atrium. Its walls are covered by ridges called the musculi pectinati (pectinate muscles), which fan out from the crista like the “teeth of a comb.” These ridges are also found in the right auricle, which is an ear-like, conical, muscular pouch that externally overlaps the ascending aorta.
An additional structure in the right atrium is the opening of the coronary sinus, which receives blood from most of the cardiac veins and opens medially to the opening of the inferior vena cava. Associated with these openings are small folds of tissue derived from the valve of the embryonic sinus venosus (the valve of the coronary sinus and the valve of inferior vena cava, respectively). During development, the valve of the inferior vena cava helps direct incoming oxygenated blood through the foramen ovale and into the left atrium.
Separating the right atrium from the left atrium is the interatrial septum, which faces forward and to the right because the left atrium lies posteriorly and to the left of the right atrium. A depression is clearly visible in the septum just above the orifice of the inferior vena cava. This is the fossa ovalis (oval fossa), with its prominent margin, the limbus fossa ovalis (border of the oval fossa).
The fossa ovalis marks the location of the embryonic foramen ovale, which is an important part of fetal circulation. The foramen ovale allows oxygenated blood entering the right atrium through the inferior vena cava to pass directly to the left atrium and so bypass the lungs, which are nonfunctional before birth.
Finally, numerous small openings—the openings of the smallest cardiac veins (the foramina of the venae cordis minimae)—are scattered along the walls of the right atrium. These are small veins that drain the myocardium directly into the right atrium.
In the anatomical position, the right ventricle forms most of the anterior surface of the heart and a portion of the diaphragmatic surface. The right atrium is to the right of the right ventricle and the right ventricle is located in front of and to the left of the right atrioventricular orifice. Blood entering the right ventricle from the right atrium therefore moves in a horizontal and forward direction.
The outflow tract of the right ventricle, which leads to the pulmonary trunk, is the conus arteriosus (infundibulum). This area has smooth walls and derives from the embryonic bulbus cordis.
The walls of the inflow portion of the right ventricle have numerous muscular, irregular structures called trabeculae carneae (Fig. 3.71). Most of these are either attached to the ventricular walls throughout their length, forming ridges, or attached at both ends, forming bridges.
A few trabeculae carneae (papillary muscles) have only one end attached to the ventricular surface, while the other end serves as the point of attachment for tendon-like fibrous cords (the chordae tendineae), which connect to the free edges of the cusps of the tricuspid valve.
There are three papillary muscles in the right ventricle. Named relative to their point of origin on the ventricular surface, they are the anterior, posterior, and septal papillary muscles:
The anterior papillary muscle is the largest and most constant papillary muscle, and arises from the anterior wall of the ventricle.
The posterior papillary muscle may consist of one, two, or three structures, with some chordae tendineae arising directly from the ventricular wall.
The septal papillary muscle is the most inconsistent papillary muscle, being either small or absent, with chordae tendineae emerging directly from the septal wall.