Question

Sinus venosus of the frog is formed by the union of
A. Two precavals and two post cavals
B. One precaval and two post cavals
C. One precaval and one post caval
D. Two precavals and one post caval

Answer 1

The auricles of the frog’s heart are two-chambered but ventricles are single-chambered. Two additional chambers are present that are called sinus venosus and truncus arteriosus.

Complete answer:
The heart of the frog is a dark reddish colored triangular organ. The two auricles and one ventricle form the three-chambered heart of the frog. The anterior side of the heart is broader and the posterior end is pointed. The auricles are situated in the anterior broader part and the ventricle is situated in the posterior pointed end. The left and the right auricles are divided by a faint longitudinal groove called an intra auricular groove. Externally it appears as one but it is two-chambered internally. The ventricle forms the most important part of the heart. It has thick muscular walls and is clearly separated by the coronary sulcus from the two auricles. The additional chamber called sinus venosus is made up of two precavals on the dorsal side. These two precavals are fused with one post-caval and form a wide chamber. It is a dark-colored triangular structure. It has thin walls that open in the right auricle. Impure blood from all body parts is poured into the sinus venosus. It further pours this impure blood into the right auricle.
So, we can conclude that the right answer is option D.

Note: The second additional chamber of the frog heart called truncus arteriosus is a tubular structure. It originates from the right side of the ventricle. It extends over the right auricle and then divides into two branches. These branches form the aortic trunk.