The frog heart has three chambers, two atria that take blood from the body and one ventricle to pump it out, according to the University of Wisconsin-La Crosse's Zoo Lab. It relies on spiral valves to move the blood to the right places. Because there is only one ventricle, there is some mixing of oxygenated and deoxygenated blood, but a groove through the ventricle helps minimize it.
A frog's heart is fairly typical of amphibian hearts, as well as those of most reptiles. It is less efficient than the four-chambered hearts of crocodiles, birds and mammals. The spiral valve that directs blood flow from the heart is able to direct deoxygenated blood to the skin and lungs and oxygenated blood to the rest of the body despite both types being pumped from one chamber.
The skin of frogs and other amphibians is unlike that of reptiles, mammals and birds in that it is a major site of gas exchange and is very thin with blood vessels very close to the surface. This use of skin to breathe is a large part of why amphibians must stay moist. It also allows some breathing underwater. Frogs' lungs resemble those of mammals, but they have no rib muscles or diaphragms to power breathing. Instead, they use mouth action to push air into their lungs.