Describe the circulatory system in a fish.
A fish has a two-chambered heart with one auricle and one ventricle. Since both the auricle and ventricle remain undivided, only deoxygenated blood passes through it. The deoxygenated blood passes from the ventricles into the gills for oxygenation. The oxygenated blood from the gills is then passed to the body parts of the fish where oxygen is utilised and carbon dioxide enters it, thereby making the blood deoxygenated. The deoxygenated blood then returns to the heart to be pumped again into the gills for oxygenation.
Since the blood passes through the heart of the fish only once in one complete cycle of the body, the flow of blood in a fish is known as single circulation.