The sinus venosus is the most caudal of the primitive heart chambers. By the fourth week of development, it receives blood from the three sets of veins; on each side:
- anterior and posterior cardinal veins via the common cardinal vein
- umbilical vein
- vitelline vein
These veins merge on each side to form the sinus horns which enter the sinus venosus. Cephalically, the sinus venosus is in continuity with the primitive common atrium. The communication between the two is the sinuatrial junction. Folding and enlargement of the heart shift the junction cepahlically and to the right.
From week five onwards, the following veins are obliterated:
- right umbilical
- left vitelline
- left common cardinal
There is shunting of blood to the right and the the right sinus horn enlarges. Consequently, little remains of the the left sinus horn at birth except the coronary sinus and the oblique vein of the left atrium. Conversely, the dominant right horn assumes the role of the sinus venosus. It becomes incorporated into the wall of the right atrium as its smooth part - the sinus venarum at birth.
The entrance of the sinus venosus into the right atrium is flanked by two folds, the right and left venous valves. Their edges fuse superiorly to form the septum spurium. When fusion of the right horn occurs, the left valve and the septum spurium fuse with the atrial septum. The inferior part of the right valve produces two structures in the mature heart: the valves at the orifice of the inferior vena cava and the orifice of the coronary sinus. The superior part forms the crista terminalis.