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Foramen ovale (heart)
In the fetal heart, the foramen ovale (), also foramen Botalli, ostium secundum of Born or falx septi, allows blood to enter the left atrium from the right atrium. It is one of two fetal cardiac shunts, the other being the ductus arteriosus (which allows blood that still escapes to the right ventricle to bypass the pulmonary circulation). Another similar adaptation in the fetus is the ductus venosus. In most individuals, the foramen ovale closes at birth. It later forms the fossa ovalis.
== Development ==
The foramen ovale forms in the late fourth week of gestation. Initially the atria are separated from one another by the septum primum except for a small opening in the septum, the ostium primum. As the septum primum grows, the ostium primum narrows and eventually closes. Before it does so, bloodflow from the inferior vena cava wears down a portion of the septum primum, forming the ostium secundum. Some embryologists postulate that the ostium secundum may be formed through programmed cell death.
The ostium secundum provides communication between the atria after the ostium primum closes completely. Subsequently, a second wall of tissue, the septum secundum, grows over the ostium secundum in the right atrium. Bloodflow then only passes from the right to left atrium by way of a small passageway in the septum secundum and then through the ostium secundum. This passageway is called the ''foramen ovale''.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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