Fish are vertebrates that have two-chambered hearts consisting of a single atrium and a single ventricle. The atrium is responsible for taking in returning blood from the body, while the ventricle's duty is to pump out blood that has entered the heart.
In fish, the ventricle pumps the deoxygenated blood to the gills and the blood is given fresh oxygen from the surrounding water. Due to the simplistic design of the two-chambered heart, a lower amount of oxygen reaches body tissues at any one time in comparison to three-chambered and four-chambered hearts. Fish have lower metabolic capacity than these animals. The metabolic needs, however, of fish are not as demanding as those of warm-blooded animals.