A 6' human will have something like 2.5 psi more blood pressure at their feet when standing, compared with their head. That's not a lot of pressure, but their blood vessels still need to squeeze a little more when standing to have the same circulation as when laying down. A bat will have less than 0.2 psi of difference upright or laying down (or upside down vs laying down). That's probably not really significant enough to require additional adaptation.