In which parts of the heart does arterial blood flow, and in which venous?

In vertebrates, the heart has a left and a right side. An exception is fish, which have one heart chamber. Amphibians and reptiles have three chambers in the heart: two atria (right and left) and one ventricle. In the right – venous, and in the left atrium – arterial blood. Birds, mammals and humans have four chambers of the heart: two atria (left and right) and two ventricles (right and left). In the right atrium and ventricle – venous blood, and in the left atrium and ventricle – arterial blood.