A three chambered heart (in the case of reptiles and amphibians) have two atria and a single ventricle. Deoxygenated blood returns from teh body and goes into the right atrium. Then, the blood is moved into the ventricle to the lungs. After picking up oxygen there, and getting rid of CO2, the blood returns to the heart and into the left atrium. Going back into the ventricle a second time, the blood is utilized by the body.
