Going up will cause a back pressure, but what goes up must come down, and that coming down will cause a forward pressure that cancels out any effect gravity had in the first place. Take a loop of tube that starts and ends at about the same place, place it horizontally and pump or blow some water through, now make it vertical and do the same again. You won't notice any difference. Use any length, any shape of loop, as long as its the same both times.
Edward
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