The sinus venosus is the first chamber to recieve returning blood. In fishes, amphibians and reptiles sinus venosus is present due to which mixing of blood takes place i.e, both pure and impure blood present in same chamber. The heart of aves and mammals is four chambered i.e., two auricles and two ventricles. The right auricle receives deoxygenated blood from the body parts and sends it into the right ventricle which pumps the deoxygenated blood to the lungs for oxygenation.