Final answer:
The main blood propulsive force in fish comes from the ventricle, the chamber of the heart responsible for pumping blood to the gills for oxygenation and then to the rest of the body.
Step-by-step explanation:
In fish, the chamber of the heart that serves as the source of the main blood propulsive force is the ventricle. Fish have a two-chambered heart consisting of a single atrium and a single ventricle. The atrium receives blood that has returned from the system, and the ventricle pumps it to the gills for gas exchange in what's known as gill circulation. After gill circulation, the blood continues through the body, completing the systemic circulation, before returning to the atrium.
While other parts of the heart, such as the atrium, sinus venosus, and conus arteriosus, play roles in the heart's blood flow, they do not provide the main propulsive force. Unlike human hearts, which have four chambers with the left ventricle pumping oxygenated blood to the body and the right ventricle pumping deoxygenated blood to the lungs, fish hearts pump blood through a single circuit starting at the ventricle.
The function of the ventricle as a pump is comparable to the left ventricle in humans which pumps oxygenated blood into the aorta and throughout the body. However, due to the different circulatory systems between fish and humans, the single ventricle in fish is responsible for pumping deoxygenated blood to the gills for oxygenation and then to the rest of the body.