Parts of heart

a) Sinus venosus: The first chamber is called the sinus venosus, it is the preliminary collecting chamber. In teleosts, it is filled from two major veins called the hepatic veins and the left and right branches of the Curvierian ducts which in turn collect blood from the paired (left and right) lateral veins the inferior jugulars, the anterior cardinals, and the posterior cardinals. However, in the elasmobranchs, only one hepatic vein leads into it.

b) Atrium. From the sinus venosus the blood flows into the atrium. The atrium is the largest of the chambers and is weakly muscular. It pushes the blood, with weak contractions in the ventricle.

c) Ventricle. The ventricle is the only well-muscled chamber, nearly as large as the atrium it is the workhorse of the heart, its contractions drive the blood around the body.

d) Bulbus arteriosus: The last chamber of the fish heart is called the bulbus arteriosus in the teleosts, but the conus arteriosus in the elasmobranchs. The difference between these chambers is that the conus arteriosus of sharks and rays contains many valves while the bulbous arteriosus of bony fish contains none. Both are alike in being primarily elastic and work to reduce the pulsed nature of the blood leaving the ventricle giving it a more even, constant flow.