Due to a valve known as the septum primum, blood cannot return from the left atrium to the right.
A thin, crescent-shaped membrane called the septum primum descends from the primitive atrium's roof in the direction of the growing endocardial cushions. The single basic atrium is divided into left and right parts by it.
The septum primum first forms as a muscular ridge that expands in the direction of the endocardial cushion tissue of the growing AV valves.
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