Two-chambered Heart
- The heart of a fish has two chambers: an atrium and a ventricle. There are two other chambers that are called the sinus venosus and the bulbus arteriosus. Unoxygenated blood is moved through the sinus venosus to the atrium where it is pumped into the ventricle and out the bulbus arteriosus. Blood enters the heart, gets pumped through the gills, and out to the body.