Amphibians in the adult stage have a three-chambered heart that separates oxygenated and deoxygenated blood to a certain extent before pumping it back throughout the rest of the body, according to Sam Houston State University. Wikipedia explains that amphibians in the juvenile stage, such as tadpoles, have a circulatory system similar to that of fish, with a two-chambered heart that circulates blood through the gills to be oxygenated.
When an amphibian matures from a juvenile to an adult, it loses its gills and replaces them with lungs. Sam Houston State University explains that an adult amphibian heart has two atria: one receives blood from the body, while the other carries in oxygenated blood from the lungs. Both of these atria empty into the same undivided ventricle, so separating the blood is not a perfect process. However, the timing of the blood entering the ventricle prevents it from mixing completely. After leaving the ventricle, the blood passes through the conus arteriosis and into the truncus arteriosis, which is bifurcated and directs blood to the rest of the body.
According to the Smithsonian, some amphibians are able to enter a state of suspended animation during the winter months through a process that involves slowing or stopping their heartbeat. An example of this is wood frogs, which survive freezing temperatures by sending sugar molecules through their circulatory systems. These sugar molecules act as a kind of anti-freeze, and when temperatures rise, the circulatory system of the half-frozen wood frog becomes fully functional again within a day.