What are the differences between a sheep heart and a frog heart? How do these differences show that a mammalian heart is more efficient?

The main difference between a sheep heart and a frog heart is that sheep have a four-chambered heart and frog three-chambered heart.

The heart of an amphibian, such as a frog, has three chambers, one ventricle and two atria. Blood from the ventricle travels to the lungs and skin where respiration occurs and where the blood becomes oxygenated. From the ventricle, blood also travels to the rest of the body where it becomes deoxygenated. In the ventricle, deoxygenated and oxygenated blood are mixed before being pumped out of the heart.

On the other hand, mammals (including sheep) have two atria and two completely divided ventricles. The left side of the heart pumps and receives only oxygenated blood while the right side of the heart pumps and receives only deoxygenated blood, meaning that their systemic and pulmonary systems are separated. The separation of these two systems enables mammals to deliver more oxygen to their tissues (because there is no mixing of oxygenated and deoxygenated blood). Thus, mammals' heart is more efficient.