The conventional model of the circulatory system assumes the heart pumps blood out of the ventricles through a series of progressively branching arteries of decreasing diameter transitioning to innumerable microscopic capillaries which then progressively converge as veins of increasing diameter connected to the atriums then back to the ventricles. Although the return trip through veins is assisted by skeletal muscle contractions squeezing blood through one-way valves, arteries lack valves and are assumed to be passive conduits.
The total length of blood vessels, if strung end to end, is said to be 60,000 miles for a human. Are there 60,000 miles of blood vessels in the human body?. Obviously the vast majority of this distance is from capillaries. So even if we assume skeletal contractions account for 100% of power for venous blood, that still leaves at least 30,000 miles of arteries and capillaries. We can further assume capillaries are multiply redundant and not actually in use simultaneously (regulated by arterial sphincters). But that still leaves thousands of miles of conduits for blood to traverse before reaching the veins.
How can a pump the size of a fist drive a fluid more viscous than water through thousands of miles of conduits, most of which are invisibly small? The ratio of circumference to cross-sectional area is inversely proportional to radius, so the total wall friction to overcome increases with distance from the heart. If you want to pump water thousands of miles, a fist-sized mechanical pump won't cut it. Maybe 100 of them at regular intervals would do it, but not one.
OK after typing all that I just realized I neglected gravity. However, blood circulates whether the person is standing, lying down, in outer space, or standing on their head. How could all of the power needed to move blood through thousands of miles of conduits be provided by such a small muscle, even at rest?
I think skeletal muscles must be the primary driver of circulation, and the heart more of a flow rate regulator.