Why does the fish has an inefficient heart?

Fish have a two-chambered heart, consisting of one atrium and one ventricle, which limits their ability to efficiently separate oxygenated and deoxygenated blood. This design means that all blood passes through the heart only once in a complete circuit, resulting in lower blood pressure and reduced oxygen delivery to tissues compared to the more complex three- or four-chambered hearts found in mammals and birds. While this is efficient for their aquatic environment, it constrains their overall metabolic capacity.