The primary difference between a frog's circulatory system and a human's is that a frog's heart has three chambers as opposed to two. Frogs are also able to breathe partially through their skin, according to the University of Wisconsin, La Crosse. Due to the ability to take in oxygen from both the lungs and the skin, a frog's heart sends blood that needs oxygen outward toward the skin and lungs from the right atrium through pulmocutaneous arteries. Once there, the blood picks up oxygen molecules and gets transported back to the left atrium.
The newly oxygenated blood in the left atrium is then either sent out through various veins to the extremities and organs or dumped into the ventricle to be mixed with poorly oxygenated blood. The ventricle mixes the blood to create a very pure blend that gets pushed to the brain and sent back to the right atrium for the process to start over.