Are the atria the top two chambers of the heart?

The atria are indeed the top two chambers of the heart in mammals, including humans. The heart consists of four chambers: two atria and two ventricles. The atria are positioned above the ventricles and are responsible for receiving blood from the body and the lungs. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs. This anatomical arrangement is consistent across mammals, which is why the answer is true.

In discussing the other choices, stating that it is false would imply that the atria are not positioned at the top, which is incorrect. The idea of "sometimes" suggests that atrial positioning could vary, but this is not accurate in standard vertebrate heart anatomy. The option regarding certain animals is also misleading since, while there are differences in heart structures among various species, the basic layout of the atria being the top chambers holds true for mammals.