<P> In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues . This is known as single cycle circulation . The heart of fish is, therefore, only a single pump (consisting of two chambers). </P> <P> In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps . Amphibians have a three - chambered heart . </P> <P> In reptiles, the ventricular septum of the heart is incomplete and the pulmonary artery is equipped with a sphincter muscle . This allows a second possible route of blood flow . Instead of blood flowing through the pulmonary artery to the lungs, the sphincter may be contracted to divert this blood flow through the incomplete ventricular septum into the left ventricle and out through the aorta . This means the blood flows from the capillaries to the heart and back to the capillaries instead of to the lungs . This process is useful to ectothermic (cold - blooded) animals in the regulation of their body temperature . </P> <P> Birds, mammals, and crocodilians show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four - chambered heart of birds and crocodilians evolved independently from that of mammals . </P>

The first blood vessel of the systemic circulation is