Simple solution...don't think too much.
i got a fantastic analogy:
your bladder's capacity: ampere*hour (AH)
the piss: current (ampere) aka "the yellow flow", "mellow yellow", etc. etc.
the pressure difference between the bladder and the outside world: voltage (V). squeeze that secret muscle and the pressure increases, etc. etc.
the blockage: impedence. not-friendly to catheters (sp)
larger the bladder, the more capacity it has, therefore, more current it will provide for a longer amount of time.
i got some nice illustrations if you folks want some
hehehehe,
marc