A fish has a heart, which acts as the center of its circulatory system, pumping blood throughout the body and transporting vital nutrients and oxygen to cells. The size, placement and strength of a fish's heart differs between species.
Fish hearts are typically two chambered. The atrium receives deoxygenated blood before sending it to the ventricle, where it is pumped back into the body of the fish. Additionally, fish hearts have two accessory chambers, which are less powerful than the two true chambers. The sinus venosus sits at the entry to the atrium and collects excess blood. The outflow tract lies on the outside of the ventricle, where it sometimes assists in the delivery of blood to the aorta for redistribution.