Structure

The amphibian heart is structurally different to that of the mammal in that it has a single ventricle in which blood returning from the lung is mixed with venous blood from the tissues. Venous blood enters at the sinus venosus, which contracts to force blood into the right atrium. If the heart is slowed by addition of cold Ringer's solution it is possible to observe that the sequence of contractions is: sinus venosus, atria, ventricle and truncus arteriosus, the latter being a contractile portion of the vessel corresponding to the mammalian aorta.