ok so all of these answers talking about gravity having less influence on bats are ignoring physics and how the actual physiology of how blood flow works. Acceleration down from gravity is the same regardless of mass, so that doesn't matter. And it is only venous blood that pools lower down. As long as that blood can get back to the heart, blood pressure and blood flow will be maintained. just like humans, bats will have things like one way valves that will make sure blood in their veins only goes towards the heart and not pooling in their head.