Question

Sinus venosus in mammals is believed to have merged with the wall of
A. Right auricle
B. Left auricle
C. Right ventricle
D. Left ventricle

Answer 1

Collects deoxygenated blood from the bloodstream and moves it into the heart. It is also called the right atrial appendage.

Complete answer:
Sinus venosus is the first chamber in the heart of many reptiles, amphibians and also in the embryo of birds and mammals. It also receives blood from the veins.
The sinus venosus is the large quadrangular cavity which precedes the atrium on the venous side of the chordate heart.
In the embryonic mammalian heart, the sinus venosus is the confluence of the cardinal veins and these are located on the right side of the heart i.e., right auricle. In, mammals, it exists in the embryonic heart where it is only found between the two venae cavae and sinus venosus is described as being incorporated into right atrium where it becomes the dorsal smooth wall of right atrium and that is called as sinus venarum.
The right auricle of the mammalian heart receives blood from the pre and post cavals. Pre and post cavals are alternative names for superior and inferior vena cava and deoxygenated blood is delivered to the right auricle by vena cava.

Hence, the correct answer is option A. i.e., Right auricle.

Note: The sinus venosus is a cardiac chamber upstream of a right atrium that harbors the dominant cardiac pacemaker. The remodeling of the sinus venosus in mammals may be an adaptation that is associated with the high heart rates.