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The circulatory system in fishes is a single circuit, with blood flowing from the heart to the gills and then to the rest of the body. The heart is located a little behind and below the gills.
The typical fish heart has four chambers, however unlike mammals, blood moves through all four in sequence. Venous blood enters the sinus venosus (a thin walled sac) then flows into the atrium, followed by the ventricle (a thick walled pump). Blood then flows into the conus arteriosus (elasmobranchs) or bulbus arteriosus (teleosts) then to the gills and the rest of the body.
The heart of slow moving fishes is comparatively small, whereas active swimming species such as the Blue Mackerel have large hearts.
- Helfman G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.