In the frog's heart, sinus venosus is formed by the union of 

The sinus venosus, an expanded area located between the vena cava and the right atrium, serves as the pacemaker in amphibians like the frog. It is triangular-shaped and is present on the dorsal side of the heart. It is formed by the union of three main vena cava. They are the two pre-cavals and one post-caval.