The Encyclopedia of Earth's Amphibian Morphology and Reproduction
Walking and jumping requires a lot more energy than swimming in water. Adult amphibians must have large amounts of food and oxygen for aerobic respiration. Aerobic respiration occurs when there is abundant oxygen for the cell's use. The presence of a three-chambered heart, efficiently pumping blood, helps to ensure that all the cells get the amount of oxygen they need for making energy. You and I have a four-chambered heart, and we will talk about that soon. But for now, check out the following picture of a typical adult frog heart. (The larva stage amphibians have only a two-chambered heart.)
Take a peek at the model of the three-chambered heart at this website.
The Encyclopedia of Earth's Amphibian Morphology and Reproduction
The large bottom chamber is the very muscular ventricle. The ventricle is used for pushing, or pumping, the blood. At the top are two smaller chambers called atria (singular atrium).
Atria are smaller heart chambers used to collect blood from the blood vessels. One atrium collects oxygen-rich blood from the lungs, the other atrium collects oxygen-poor blood returning from the body. Blood moves from each atrium into the ventricle.
Even though there would seem to be a mixing of the oxygen-rich blood from the lungs and oxygen-poor blood from the body in the ventricle, the anatomy of the heart somewhat prevents the mixing, allowing the oxygenated blood to move on from the heart to the body and the oxygen-poor blood moves into the lungs. Thus, the blood of the frog moves efficiently where it needs to go so that gases can be exchanged in the lungs and the body cells are provided with oxygen.
