So, drugs and their metabolites enter the kidney tubule but some may be reabsorbed back into the blood stream. Reabsorption is referred to as passive diffusion since the process does not require energy.

This flow of drug back into the blood occurs because water is reabsorbed from the kidney tubules by diffusion. In fact, the majority of water that initially enters the nephron is reabsorbed back into the blood as a means of conserving body fluid. As this movement occurs, some drugs are transported along with it.