Between 22 and 28 days after conception, the fetal heart is transformed from a pulsatile tube into the hollow single chambered pump pictured below. Around day 28 new structures begin to form which direct and divide the flow of blood inside the heart.
The first division is accomplished by the endocardial cushions. The endocardial cushions begin the separation of the heart into right and left, upper and lower chambers. These chambers will become the atria and ventricles. While the endocardial cushions continue to develop, the atrial and ventricular septa begin to form.
The ventricular septum is composed of thick, muscular tissue derived from the walls of the growing ventricles. When this muscular septum is complete, a large opening (the interventricular foramen) still remains between the two ventricles.
The interventricular foramen is usually completely closed by week seven. Closure is accomplished by growth of membraneous tissue derived from the endocardial cushions, the interventricular septum and from the conus ridges formed within the truncus and extending to the interventricular septum.
The heart is divided into a two chambered pump by the endocardial cushions.
The Atrial Septum
The common atrium is divided by the atrial septum, also known as the septum primum. The atrial division begins around day 33, about the same time nostrils and hand plates are developing.
The atrial septum reduces the flow of oxygenated blood from the right atrium to the left. It continues to grow until the foramen primum is closed. However, as the flow of oxygenated blood across the foramen primum almost ceases, a focal area of cellular death occurs in the atrial septum causing an opening to form called the foramen secundum. The foramen secundum allows for continued delivery of oxygenated blood to the left atrium.
Septum secundum begins forming about day 38 and is complete by day 45. It is thicker than the atrial septum and overlaps the foramen secundum, creating the foramen ovale. The foramen ovale establishes a one-way passage for blood from the right atrium to the left. Oxygenated blood from the placenta is thus directed into two streams, one across the foramen ovale to the left atrium and onward to the aorta, the other to the pulmonary vasculature.
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