Blood can only circulate in one direction through the body of a fish. Its vascular system is completely sealed up. In addition to that, there are only two chambers in the heart that make up this organ. Four separate chambers make up the heart: the bulbous arteriosus, the atrium, the ventricle, and the sinus venosus. The heart acts as a pump, bringing blood to the gills from the rest of the body. In addition, the gills are responsible for oxygenating the blood before it is distributed throughout the body.