A typical adult amphibian heart has three chambers: two atria and one ventricle. This arrangement supports amphibians’ unusual transition between aquatic and terrestrial life.
The right atrium receives oxygen-poor blood returning from the body, while the left atrium receives oxygen-rich blood from the lungs and skin. Both atria empty into the single ventricle, where some mixing can occur. Internal structures and the timing of contractions help reduce that mixing before blood is sent either to the lungs or around the body.
This is different from the two-chambered heart of a tadpole, whose circulation resembles that of a fish. Tadpoles generally use gills and a single circulation loop. As they metamorphose, they develop lungs and the adult three-chambered system.
The four-chambered heart of birds and mammals is a common distractor. It completely separates oxygen-rich and oxygen-poor blood, whereas most adult amphibians retain one ventricle. Some amphibian species also rely substantially on skin breathing.