The amphibian circulation is double because it is composed of systemic and pulmonary circulations: respectively, heart-tissues-heart and heart-lungs- heart. Since amphibians have only one ventricle in the heart, venous blood taken from the tissues and arterial blood coming from the lungs are mixed in the ventricle that then pumps the mixture back to the systemic and to the pulmonary circulations. The amphibian circulation is classified as incomplete because venous and arterial blood mix in the circuit.
The blood oxygenation in amphibians occurs also in the systemic circulation since their skin is a gas exchange organ.