81 - Hepatic First Pass and Enterohepatic Recirculation

KYJ 81- Hepatic First Pass

In this series of Knowing your Jargon (KYJ) we will look at Phamacokinetics (what our body does to drugs).

Over the next few days we look at :
Absorption - (covered KYJ79)
Transportation
Hepatic First Pass
Protein binding
Bioavailability
Half life
Distribution
Metabolism and metabolites
Excretion

In this episode, we look at the concepts of hepatic first pass, and a related term called enterohepatic recirculation.

Now let's go on a journey. You and I are an antibiotic (say Trimethoprim). It's 10pm and Sally the patient has been diagnosed this afternoon with UTI and Dr Tinkle prescribed a three day blast of Triprim, to be commenced tonight at bedtime.

Sally switches of "Gator people" the latest in classy reality TV shows to hit the small screen, has her last whoofy stinging wee for the night. Pops the Antibiotic and goes off to sleep.

As Sally swallows you, you dissolve almost immediately. Your transit through the stomach reduces you to mere particles in a slurry as you pass into the small intestine.

Here you are absorbed through the Villi, small finger like projections that line the rugated mucosa of the bowel.  Absorbed into the blood supply, your very next port of call is the Liver.  There you are metabolised into other molecular compounds. This movement from gut to liver is called Hepatic First Pass.  It literally means that all ingested medicines are first passed through the liver before systemic circulation.

Now some drugs or their metabolites are released into systemic blood, where they exert their action by being transported to the sites of action. Some drugs or metabolites are secreted by the liver into Bile. This is manufactured in the liver and stored and concentrated in the Gall Bladder.  During Sally's next meal, fat in her food will stimulate the Gall Bladder to contract and secrete bile into the duodenum to mix with the food bolus.

Many drugs, (eg antibiotics) after hepatic metabolism are bound to bile salts, where normal bowel bacteria release them for reabsorption.  They again undergo hepatic metabolism and the cycle repeats until the drug is eventually eliminated.

This process of drug going round and round between gut and liver is called Enterohepatic Recirculation.

The drugs can be eliminated by many means, but that is a post for another day.

Hope these Pharmakokinetic KYJ posts are helping you hone your pharmacology knowledge.
If you are getting anything from these, let me know and tag a bunch of your nursing crew.
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