Talk about distal tubular reabsorption of drugs.

Drug concentration gradient in distal tubules exceeds that in the perivascular space, thus the drug if uncharged may diffuse into systemic circulation. To control this we may manipulate urine pH to get drugs to become charged to be eliminated, so weak acids are eliminated by alkalinization of urine, and weak bases are eliminated by acidification of urine (this is ion trapping) ex: phenobarbital overdose (weak acid) we alkalinize the urine to eliminate it