 Amphibians have a juvenile stage and an adult stage , and the circulatory systems of the two are distinct . In the juvenile ( or tadpole ) stage , the circulation is similar to that of a fish ; the two @-@ chambered heart pumps the blood through the gills where it is oxygenated , and is spread around the body and back to the heart in a single loop . In the adult stage , amphibians ( especially frogs ) lose their gills and develop lungs . They have a heart that consists of a single ventricle and two atria . When the ventricle starts contracting , deoxygenated blood is pumped through the pulmonary artery to the lungs . Continued contraction then pumps oxygenated blood around the rest of the body . Mixing of the two bloodstreams is minimized by the anatomy of the chambers . 
