part_of part of part of develops from immediate transformation of sinus venosus http://upload.wikimedia.org/wikipedia/commons/c/c7/Gray465.png In fish, receives venous blood from the systemic circulation through paired hepatic veins, anterior jugular veins and Cuvierian ducts[PMID:20735616] In humans, it exists distinctly only in the embryonic heart, where it is found between the two venae cavae. In the adult, it is incorporated into the wall of the right atrium to form a smooth part called the sinus venarum, also known as the venarum sinus, which is separated from the rest of the atrium by a ridge of fibres called the crista terminalis. The sinus venosus also forms the SA node and the coronary sinus. 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. It 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 and oblique vein of the left atrium, whereas the right part becomes incorporated into the right atrium to form the sinus venarmu conducting system of heart cardiac chamber venous sinus presumptive sinus venosus