Final answer:
The main force for the bulk flow of blood is the contraction of ventricles, which creates high pressure during ventricular systole, supplemented by the elastic recoil of arteries to maintain pressure gradients.
Step-by-step explanation:
The Force Behind Bulk Flow of Blood
The primary force for the bulk flow of blood is the contraction of ventricles during ventricular systole. When the ventricles contract, they create high pressure that forces blood out of the heart and into the arteries. This is further supported by the elastic recoil of arteries, which helps maintain the pressure gradient necessary for blood flow. While the contraction of atria and valves in veins play roles in the cardiac cycle, they do not provide the main force for blood ejection and circulation.