79 - Drug Absorption (Pharmacokinetics series)

KYJ 79- Drug Absorption

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
Transportation
Hepatic First Pass
Protein binding
Bioavailability
Half life
Distribution
Metabolism and metabolites
Excretion

In this session: Absorption and transport of a drug in our bodies.

Consider three different drugs, orally ingested Warfarin. Intravenous Morphine and a topical tinea (ringworm/athletes foot) cream like Miconizole.  All different drugs yet totally different routes of administration.

Absorption refers to the way that a drug gets from where we give it, to the site in the body where the drug will exert its influence.  For most drugs, the drug is carried in the blood an exception would be a topical drug delivered directly to the site of action eg the tinea cream.  When a person has a fungal lesion, Miconizole cream can be applied directly to affected skin where it exerts it's action topically. It is not absorbed into the blood stream.
Other examples include ear or eye drops, even salbutamol given by nebuliser or metered  aerosol, is delivered topically at the site in smooth muscular  bronchiole tissue, where is causes smooth muscle relaxation.

Systemic absorption
Some topical drugs are absorbed into the skin where it diffuses into the capillaries and enters the circulating blood. Examples include GTN (Transiderm) patches, Fentanyl and nicotine patches.  Steroids like oestrogen, progesterone, hydrocortisone and non-steroid  anti-inflammatory rubbing gels are systemically absorbed into the blood, eg Diclofenac (Voltaren Emulgel), and Ibuprofen.

When a drug is administered directly into the blood, it doesn't need absorption. These drugs are called parenteral drugs. Any drug given intravenously, intramuscular or subcutaneously fall into this category. Once injected, intravenous morphine is immediately into the blood stream and available to be carried to the brain where it binds to Mu receptors and exerts it's action centrally to inhibit pain sensation.  In recent years other drugs eg Ondanzetron and Midazolam have been administered into the mouth or nose where they are absorbed directly into systemic blood through a dense network of blood vessels.

Other drugs are slower to be absorbed.  When swallowed the medication packaged usually in a pill (sugar film coated) , capsule (protein film coated), or tablet (compressed powder), is swallowed, where it is broken down and digested.  The drug is absorbed from blood vessels lining the stomach or most commonly the small intestine.   Once entering the gut blood, this drug is transported directly to the liver and metabolised.  This is called Hepatic First Pass (HFP).  More on HFP in a later session.

For many drugs this HFP destroys the drug, making some drugs less active when taken orally (Pethidine, diazepam) or completely inappropriate for oral ingestion (eg insulin, heparin, GTN), for other drugs, this hepatic metabolism is necessary to metabolise a drug into an active chemical that exerts an action.
An example is Metronidazole (Flagyl) , paracetamol (Panadol), Enalpril and even codeine all need conversion into an activated new chemical to do their job.

Absorption is how a drug enters the blood stream. In our next episode, we will look at transportation and protein binding.

Now diarise a few notes and reflect on this revision - all our KYJs are claimable for 0.25 CPD for RNs , ENs and Scheduled Medicines endorsed RNs;  but you have to make reflective notes and keep as evidence.

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• uses relevant literature and research findings to improve current practice

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