The chambers and subchambers of roe fish open unidirectionally. Never opens backwards. Flows unidirectionally from the back of the body through the heart. For this reason it is called venous heart. It is called venous heart because it only carries blood containing CO2.Dr. Siddiq Publications
Blood from different parts of the body enters the sinus venosus through veins, ductus cuveae and hepatic sinuses. The sinus venosus contracts and blood enters the atrium. When the atria are full of blood, they contract and blood enters the ventricles. When the blood-filled ventricles contract, blood moves into the bulbous aorta. Blood enters the gills from the bulbous aota.