Loyola University Medical Education Network Formation of the Ventricles

Fate of the Sinus Venosus (Formation of the Right Atrium)

Figure 1

Unlike the atria, the sinus vinosus remains a paired structure with right and left horns. Each horn receives venous blood from three vessels:

Communication between the sinus venosus and the primitive atrium, the sinoatrial oriface, is centrally located.

Figure 2

Gradually the sinoatrial oriface shifts to the right, due to the shunting of blood to the right, until the sinus venosus communicates with only the right atrium. The fate of each structure is as follows:

Conversely, the left vein counterparts are obliterated and the left sinus horn diminishes in size and forms the coronary sinus and the oblique vein of the left ventricle.

Figure 3

Internally, the sinoatrial oriface is flanked by two valves, the right and left venous valves. Superiorly these two valves meet to form the septum spurium. Note that the left horn opens up underneath the oriface of the right horn (sinoatrial oriface). This is the orifice of the coronary sinus.

Figure 4

Further into development the right sinus horn is incorperated into the expanding right atium. As the atrium expands the smooth tissue of the sinus venosus displaces the trabeculated tissue of the primitive right atium anteriorly and laterally where it becomes the adult right auricle. The smooth tissue forms part of the atium called the sinus venarum. Crista Terminalis, a ridge of tissue located to the right of the sinoatrial oriface, forms the boundry between the auricle and the sinus venarum.

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