If you look at a diagram of a toad’s circulatory system, you will notice the heart is different than ours. It has two atria and a single ventricle, making it have a total of three chambers. Like all other land animals, the toad has double circuit circulatory system. Blood is circulated through successive contraction and relaxation of heart.
The two anterior vena cavae and the posterior vena cava carry the carbon-dioxide enriched blood into the sinuis venosus. This blood enters the right auricle through the sinu-auricular aperture Almost simultaneously the right auricle relaxes and from the lungs oxygen enriched blood enters into the left auricle through the pulmonary vein. Right and left auricles simultaneously contracts and from the right and left auricles carbon dioxide and oxygen enriched blood through the common auriculo-ventricular aperture enters the cavity of the ventricle.
Later, the wall of the ventricle contracts and through the control of the semilunar valve, blood from the ventricle enters the cavity of the cons arteriosus. Afterwards oxygenated and carbon-dioxide enriched mixed blood through the right and left aorta circulates respectively through the carotid, systemic and pulmocutaneous arteries.