sinus venosus sinus venosus the sinus venosus is a large quadrangular cavity which precedes the atrium on the venous side of the chordate heart. this article is about the embryological structure. for the heart defect, see atrial septal defect § sinus venosus atrial septal defect. sinus venosusinterior of dorsal half of heart from a human embryo of about thirty days, frontal view. (opening of sinus venosus labeled at center top.)human embryo with heart and anterior body-wall removed to show the sinus venosus and its tributaries. (sinus venosus labeled at center left.)detailscarnegie stage9systemcardiovascular systemidentifierslatinsinus venosus cordista98a12.0.00.016ta23911tevenosus_by_e5.11.1.3.2.0.4, e5.11.1.5.1.0.1 e5.11.1.3.2.0.4, e5.11.1.5.1.0.1 fma70303anatomical terminology in mammals, the sinus venosus exists distinctly only in the embryonic heart where it is found between the two venae cavae; in the adult, the sinus venosus becomes incorporated into the wall of the right atrium to form a smooth part called the sinus venarum which is separated from the rest of the atrium by a ridge called the crista terminalis. in most mammals, the sinus venosus also forms the sinoatrial node and the coronary sinus. development 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 diagram to illustrate the simple tubular condition of the heart. heart of human embryo of about fourteen days. scheme of arrangement of parietal veins.