The heart of the frog has three chambers, consisting of one ventricle and two atria . Blood leaves the heart from the ventricle through a single truncus arteriosus which is short and soon branches into two aortic arches which loop left and right and dorsal to the heart to rejoin as a single aorta in the mid dorsal region of the body cavity. Each aortic arch has a branch leading to the lungs and skin where oxygenation occurs. Carotid arteries also branch off the aortic arches and supply the head region.
Veins bring blood to the left and right atria.
Both atria then empty into the single ventricle. Blood from the ventricle thus enters either the pulmonary or body circulation. Frog disection image 5 Since there is only a single ventricle there is some mixing of oxygenated and deoxygenated blood. This mixing is not complete since there is a special valve system which partially keeps the two separate. The circulatory system of animals such as the frog in which a three chambered heart is present and in which oxygenated and deoxygenated blood mix before leaving the heart is called an incomplete double circulatory system .