The blood of the fish serves, as does the blood of other vertebrates, to transport oxygen, nutrients, and wastes.

A 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 where as mammals have a four-chambered heart consisting of two atria and two ventricles.