A number of internal specializations occur during the development of the embryonic heart: The common atrium is partitioned into right and left chambers between 4-7 weeks (Table 2.03). This is accomplished by a septum, interatrial septum I or septum primum, growing down from the roof of the atrium toward the A-V canal. A hole develops in it, allowing blood to continue flowing from right to left. This is interatrial foramen II.
Next, a second septum begins growing down from the roof of the atrium in the same way, on the right side of septum I. This is septum secundum. It also develops a hole which becomes the foramen ovale (ovale window), but this one is out of register with the first hole. Blood continues to flow from right to left just as long as the pressure in the right atrium is higher than that in the left atrium.
Pressure reversal, as occurs at birth, causes the lower flap of Septum I to cover the foramen ovale, preventing backflow. Actually, all of the venous blood doesn't flow into the right atrium; some fraction is directed by the crista dividens through the foramen ovale, over a path known as the via sinistra, largely bypassing the right atrium (Figure 2.08).. This minimizes the mixing of the more oxygenated blood from the lower body (placenta via inferior vena cava) and less oxygenated blood from the head and upper body (via superior vena cava).
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