How do paralyzed people from the waist down go to the bathroom?

It depends on how they became paralyzed. If your talking a Spinal chord Injury, then usually a catheter must be used to empty all of the urine from the bladder. I think it has to do with the sudden neurological trauma received. just google
'SPINAL CHORD INJURY'S" you'll find all sorts of cool and fun tidbits. (I'm paralyzed myself btw).