Fish circulation

CIRCULATION

The blood of the fish serves, as does the blood of other vertebrates, to transport oxygen, nutrients, and wastes. The typical fish's circulation is a single circuit: heart-gills-body-heart. In contrast, mammals have two circuits: heart-lungs-heart and heart-body-heart. The fish heart proper is two-chambered, consisting of an upper atrium and a lower ventricle. Amphibians, basically, have a three-chambered heart, two atria and one ventricle; reptiles have a three- or four-chambered heart; and mammals and birds have a four-chambered heart consisting of two atria and two ventricles. The fish heart, however, has two accessory chambers, and all four chambers are contained within a single pericardial sac. One accessory chamber is the thin-walled sinus venosus, which collects blood and leads into the atrium; the other accessory chamber is the conus arteriosus, an enlargement of the main artery leading out of the ventricle. In some fishes, such as sharks, the conus arteriosus is muscular and pumps blood in the manner of the ventricle.