The heart as a biological Ram Pump
A ram pump is a not new invention that takes the downward pressure of a fluid exerted by gravity and returns a higher pressure, allowing a fluid to be pumped up an incline for a not insignificant distance. This is possible due to the magic of a compact series of check valves that quickly alternate open/closed, combined with a small elastic bladder.
But what if the heart is nothing other than an inverted ram pump? In other words, it takes the downward pull of a fluid, and converts that into an even greater downward push, pushing a fluid(blood) even greater distances. This would explain why bed ridden individuals can see a very quick decrease in muscle mass (15% reduction over a 72 hour hospitalization). If the system does not have a downward fluid pull it lacks sufficient pressure to deliver fluid/pressure based nutrients to the body.