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→Excretion
==Excretion==
==Excretion==
Drugs are mainly excreted in either the urine or bile.
===Biliary Excretion and Enterohepatic Recycling===
Liver cells can transfer drugs from the hepatocytes and plasma into bile. Some drug conjugates (mainly glucuronides) are concentrated in the bile and transported to the intestines. Active drug is then released here, any free drug is then re-absorbed and recirculates around the body. Eventually the drug will leave the body in faeces.
===Renal Excretion===
This is mainly a passive process and isn't very species specific. Though urine pH will greatly affect the extent of excretion of some drugs.
* The majority of drugs, unless protein bound, are filtered readily.
* Weak acids and bases are actively secreted into the renal tubule and are rapidly excreted.
* Lipid-soluble drugs are passively reabsorbed across the tubule and so aren't excreted readily by the urine.
* Due to ion trapping weak acids are more easily excreted in alkaline urine, and visa versa.
* A lot of drugs are mainly excreted by renal excretion and so toxicity can result if a patient has renal disease.
==Quantative Pharmacokinetics==