The heart of a fish typically has two chambers: one atrium and one ventricle. This simpler structure is quite distinct from the four-chambered hearts found in mammals. In fish, the blood flows in a single circuit, meaning it travels from the heart to the gills for oxygenation and then directly to the rest of the body before returning to the heart.
In more detail, the atrium receives deoxygenated blood from the body and pumps it into the ventricle, which then moves the blood to the gills. Here, the blood gets oxygenated and is then circulated to the fish’s tissues. This two-chambered system is efficient for their aquatic environment, ensuring that the fish can efficiently manage its oxygen needs.