The excretion of drugs by the kidney utilises three processes, all which occur in the nephron, the microscopic functional unit of the kidney. These processes are:

  1. Glomerular filtration,
  2. Tubular secretion and
  3. Tubule reabsorption.

Drugs and/or their metabolised products are transported by the capillaries to the kidney tubule. Some drugs enter the tubule by glomerular filtration at the renal corpuscle. This acts like a sieve allowing small drugs and those not bound to plasma protein to filter from the blood into the Bowman’s capsule.

Large drugs like heparin or those bound to plasma-protein cannot be filtered and are poorly excreted by glomerular filtration.