What purpose is served by the pulmonary arteries mammals have versus those of amphibians?

Question

Compare and contrast a mammal’s blood circulation and a frog’s blood circulation that results in only deoxygenated blood in the pulmonary arteries of a mammal but partially oxygenated blood in the pulmonary arteries of a frog. What purpose is served by the pulmonary arteries mammals have versus those of amphibians?

Answer 1

The key difference between a mammalian circulatory system and an amphibian circulatory system is the number of chambers in the heart. Mammals have four chambers (two atria and two ventricles) whereas amphibians only have three chambers (still two atria, but only one ventricle).

This means that in amphibians, oxygenated blood returning into the heart from the pulmonary circuit (lungs and skin) mixes with deoxygenated blood returning to the heart from the systemic circuit (which supplies blood to other body tissues).

The oxygenated and deoxygenated blood mix together in the single ventricle, which pumps the resultant partially oxygenated mixture to both the lungs/skin and the rest of the body. That is the reason for the partially oxygenated blood in the pulmonary arteries of a frog.

The pulmonary arteries in mammals and amphibians both serve the same basic purpose; they carry blood to the lungs (and skin, in amphibians), pump it up with oxygen, and send it back to the heart through the pulmonary veins. The only difference is that in mammals, the blood sent to the lungs is "pure" low oxygen blood straight from the right ventricle. In amphibians, the blood sent to the lungs and skin is mixed blood, coming both from the systemic circuit and the pulmonary circuit. Because of the additional chamber separating high oxygen blood and low oxygen blood, the mammalian heart is significantly more efficient than the amphibian heart.