The thin-walled atria receive blood coming into the heart, whereas the relatively thick-walled ventricles pump
blood out of the heart.
More force is required to pump blood through the body than through the lungs, so the muscular wall of the left ventricle is thicker than the right.
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| Fig. 1 The Heart has two pumps |
Right Atrium
In the anatomical position, the right border o f the heart is formed by the right atrium. This chamber also contributes to the right portion of the heart's anterior surface.
Blood returning to the right atrium enters through one of three vessels. These are:
- the superior and inferior venae cavae, which together deliver blood to the heart from the body; and
- the coronary sinus, which returns blood from the walls of the heart itself.
The superior vena cava enters the upper posterior portion of the right atrium, and the inferior vena cava and
coronary sinus enter the lower posterior portion of the right atrium.
From the right atrium, blood passes into the right ventricle through the right atrioventricular orifice. This opening faces forward and medially and is closed during ventricular contraction by the tricuspid valve.
The interior of the right atrium is divided into two continuous spaces. Externally, this separation is indicated by a shallow, vertical groove (the sulcus terminalis cordis) , which extends from the right side of the opening of the superior vena cava to the right side of the opening of the inferior vena cava. Internally, this division is indicated by the crista terminalis (Fig. 2) , which is a smooth, muscular ridge that begins on the roof of the atrium just in front of the opening of the superior vena cava and extends down the lateral wall to the anterior lip of the inferior vena cava.
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.
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