Fishes have 2 chambered heart, one atrium and one ventricle. Deoxygenated blood is pumped to the gills by heart where blood is oxygenated by diffusion. The oxygenated blood from the gills flows to rest of the body parts where oxygen is picked up and carbon dioxide returned back to the blood. The deoxygenated blood returns back to heart by passing through heart only once in the complete circulation.