The question above is incorrect, as circulation in a frog is accomplished via a three-chambered heart which is the pumping mechanism of its circulatory system. Oxygenated blood from the lungs and deoxygenated blood from the tissues enter two separate upper chambers of the heart known as atria. The blood from each atria passes down a valve to the ventricle, where mixing of oxygenated and deoxygenated blood does occur although it is minimized. This is due to the fact that oxygenated blood travels to the aorta to be transported throughout the body and deoxygenated blood travels to the pulmonary artery to the lungs. A three chambered heart is seen in Amphibians and in most Reptiles, excluding the Crocodilians.