In embryo, the cavity caudad of cardiac tube where the veins from the intra- and extraembryonic circulatory arcs meet, and receives blood from the vitelline vein, umbilical vein, and common cardinal vein
In mammals, the sinus venosus is distinct in the heart of an embryo. As the embryo develops, the sinus venosus forms a part of the right atrium called sinus of the vena cava.
In other chordates such as fish, amphibians, and reptiles, the sinus venosus persists in adult. It is the chamber receiving blood from veins, and then passing blood into the atrium.