The fish heart - the pump

The heart is the pump that generates the driving pressure for the circulation of blood (P1 = the arterial pressure in the previous pages). The fish heart has one atrium and one ventricle; this is in contrast to the human (mammalian) heart that has two separate atria and two separate ventricles. In the fish heart, two other chambers can also be found: the sinus venosus and the bulbus arteriosus

 

The blood from the body, which is low in oxygen enters the atrium via the sinus venosus, which contains the pacemaker cells that initiate the contractions. The blood is pumped into the ventricle by the atrium, which is a thin-walled muscular chamber. Then the blood is pumped out into the bulbus arteriosus by the ventricle: a thickwalled chamber with lots of cardiac muscle. The ventricle is responsible for the generation of the blood pressure. The last chamber, the bulbus arteriosus, is a unique structure and one of the functions is to dampen the pressure pulse generated by the ventricle. Why? The next organ after the bulbus arteriosus are the gills, and they are thin walled and may be damaged if the pulse pressure (or absolute pressure) becomes to high. The bulbus arteriosus contains elastic components but not many muscle fibres.