The heart in fish only has two chambers, but oxygenated and deoxygenated blood never mix. Why is the 3-chamber heart of amphibians considered a better circulatory system even though oxygenated and deoxygenated blood do mix?
a. Because fish do not have lungs and thus do not need to have a second loop in the circulatory system
b. Because amphibians have cutaneous respiration, they need to be able to pump blood to the skin
c. The blood in amphibians contains more oxygen, thus it requires a stronger heart in order to pump it through the body
d. In fish, blood loses pressure as it goes from the gills to the tissues withous passing again through the heart
e. Amphibians are endotherms requiring high energy expenditure, faster heart rates and oxygen levels.

In a fish, the heart comprises two chambers, a ventricle, and an atrium. The deoxygenated blood comes through the body, moves into the atrium and then into the ventricle, from where it is pushed towards the body through the gills.

The pumping of the blood via the heart takes place via the capillary beds of the gills and via the tissues’ capillary beds. When the blood come in the capillary beds of the gills it gets slow down. It moves gradually via its path to the tissues and requires skeletal muscular activity to help the blood to mediate towards the heart again. Thus, a fish is dependent upon the physical effort for blood to get circulated again to the heart.

The heart of an amphibian-like the frog exhibits three chambers, two atria and one ventricle. In the process of circulation, that is, in the ventricle, the oxygenated and the deoxygenated blood gets mixed prior to being impelled out of the heart. However, amphibians possess lower metabolisms and thus, they need less oxygen. The requirement of a lesser amount of oxygen puts less pressure on the heart to provide blood with high oxygen. Thus, a heart comprising three chambers is suitable for the requirements of amphibians who could absorb oxygen via their moist skin.