Loyola University Medical Education Network Atrial Partitioning

Atrial Partitioning

Figure 1

By the time the heart tube has formed the bulboventricular loop , the two primitive right and left atria have fused to form a common atrium. Note that it now lies cranial to the primitive ventricle and dorsal to the bulbus cordis. The truncus arteriosus lies on the roof of the common atium causing a depression and indicates where septation of the atrium will occur.

Figure 2

The partitioning of the atrium begins with the appearance of septum primum at about the 28th day. This is a crest of tissue that grows from the dorsal wall of the atrium towards the endocardial cushions - - the ostium (opening) formed by the free edge of septum primum is the ostium primum.

Figure 3

Before the septum primum fuses with the endocardial cushions, perforations appear in the upper portion of the septum primum. These perforations will coelasce to form the ostium secundum.

Figure 4

Unlike the septum primum, septum secundum does not fuse with the endocardial cushions. Its free edge forms the foramen ovale. The left venous valve and the septum spurium, located on the dorsal wall of the right atrium, fuse with the septum secundum as it grows.

Figure 5

At the end of the seventh week the human heart has reached its final stage of development. Because the fetus does not use its lungs, most of the blood is diverted to the systemic circulation. This is accomplished by a right to left shunting of blood that occurs between the two atria.

The foramen ovale and the septum primum control this right and left communication. The septum primum acts as a valve over the foramen ovale. At birth the child will use its lungs for the first time and consequently more blood will flow into the pulmonary circulation. The pressure increase in the left atrium (where the pulmonary veins empty) will force septum primum to be pushed up against septum secundum. Shortly thereafter the two septa fuse to form a common atrial septum.

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John A. McNulty
Last Updated: April 14, 1996
Created: September 25, 1995