In the embryo, the thin walls of the sinus venosus are connected below with the
right ventricle, and medially with the
left atrium, but are free in the rest of their extent. It receives blood from the
vitelline vein,
umbilical vein and
common cardinal vein. The sinus venosus originally starts as a paired structure but shifts towards associating only with the right atrium as the embryonic heart develops. The left portion shrinks in size and eventually forms the
coronary sinus (right atrium) and
oblique vein of the
left atrium, whereas the right part becomes incorporated into the
right atrium to form the
sinus venarum. ==Additional images==