Only about 1% of the glomerular filtrate actually leaves the body because the rest (the other 99%) is reabsorbed into the blood. Tubular reabsorption is the process by which solutes, drugs, metabolites and water are removed from the tubular fluid. These transport processes are mostly driven by diffusion and upon occasion, active transport.
In the distal tubule the membrane is readily permeable to lipids so filtered lipid soluble substances undergo passive diffusion down the concentration gradient. They are extensively reabsorbed and little is lost in urine.Polar drugs are unable to be reabsorbed and are voided in urine.
Many drugs are either weak bases or acids and thus the pH of the filtrate can
greatly influence the extent of tubular re-absorption for many drugs. When urine is alkaline, weak acid drugs won’t be reabsorbed. Alternatively when urine is more acidic, basic drugs are more extensively secreted in urine because the low pH favours ionisation and thus absorption is inhibited. This is called the ion-trapping effect. These changes can be quite significant as urine pH can vary from 4.5 to 8.0 depending on the diet (e.g. meat can cause a more acidic urine) or drugs (which can increase or decrease urine pH).
See how the effect of a change in pH can influence drugs moving across a membrane by clicking here.