Fish hearts differ from those of other vertebrates by having a two-chambered structure. The fish heart consists of an atrium and a ventricle, with blood pumped in a single circuit through the gills and then to the rest of the body. This simple design facilitates oxygenation in aquatic environments. In contrast, mammals, birds, and some reptiles have four-chambered hearts, featuring two atria and two ventricles. This separation enables a more efficient separation of oxygenated and deoxygenated blood, suitable for the demands of a terrestrial lifestyle with higher energy requirements and a more complex respiratory system.