Answer:
fish: blood flows unidirectionally from the two-chambered heart through the gills and then the rest of the body.
mammal: Blood is pumped from veins of the systemic circuit into the right atrium of the heart, then into the right ventricle. Blood then enters the pulmonary circuit, and is oxygenated by the lungs. From the pulmonary circuit, blood re-enters the heart through the left atrium.