Unchanged drug and active and inactive drug metabolites are mostly excreted in the urine through the processes of glomerular filtration and active tubular secretion.

Tubular reabsorption can also modify the net drug excreted (M.§.t?ke §n.d Mi!!ik§D 1992; §.§.DD.§.tí.e?,a( 1994). Drugs bound to plasma proteins cannot be filtered because of the effective glomerular filtration barrier. Only free unbound drug is filtered at the glomerular membrane in the absence of glomerular injury. The degree of plasma protein binding, rate of filtration, drug concentration, and perfusion pressure correlate with the amount of drug that is filtered. Active tubular secretion is a carrier-mediated process that takes place primarily in the proximal tubules and is dependent on the plasma concentration, renal clearance, and filtration rate of the drug. In contrast, a passive reabsorption process takes place in both the distal and proximal tubules and depends mostly on the gradients between tubular fluid and plasma (M,,at,zk.e.,.a.n.d M.iyike.n 1,9,9,2,; Bennett efa/ 1994; R.udy. .. . and . . . .Brater . . .1.994.).

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