Nephrostomy tubes?

I swore I would stay off this site, but I just find it so helpful. I had a 40 yr old male patient this week who got bil nephrostomy tubes put in because he was having all sorts of issues (retention, acute pyelonephritis, an old stricture)

I got him the day after the surgery. He had bil leg bags and a foley. Both leg bags were draining fine and the foley had only 50 cc in it all day (which I figured was normal since he was draining so much out of the tubes). I had an order to DC the foley and took it out and patient did not void throughout the night. The next morning, the renal doc asked me to bladder scan the patient. . .and he had 527cc of urine in there!

My question is. . .how can there by that much urine in his bladder if he has the tubes? Shouldn't all or at the least the majority of his urine be coming out of the tubes? I feel like an idiot. . .I guess I don't understand the mechanism behind the tubes and the renal doc isn't the type who likes to teach/explain.

Thanks, wise ones!