As we come to the vertebrates, we begin to find real efficiencies with the closed system. Fish possess one of the simplest types of true heart. A fish's heart is a two-chambered organ composed of one atrium and one ventricle. The heart has muscular walls and a valve between its chambers. Blood is pumped from the heart to the gills, where it receives oxygen and gets rid of carbon dioxide.
Blood then moves on to the organs of the body, where nutrients, gases, and wastes are exchanged.
However, there is no division of the circulation between the respiratory organs and the rest of the body. That is, the blood travels in a circuit which takes blood from heart to gills to organs and back to the heart to start its circuitous journey again. Frogs have a three-chambered heart, consisting of two atria and a single ventricle. Blood leaving the ventricle passes into a forked aorta, where the blood has an equal opportunity to travel through a circuit of vessels leading to the lungs or a circuit leading to the other organs. Blood returning to the heart from the lungs passes into one atrium, while blood returning from the rest of the body passes into the other.