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Pharmacokinetics - Elimination: Nursing Pharmacology

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Pharmacokinetics refers to the movement and modification of a medication inside the body. So once the medication is administered, first it has to be absorbed into the circulation, then distributed to various tissues throughout the body, metabolized or broken down, and finally, eliminated or excreted in the urine or feces.

You can remember this with the acronym ADME; which stands for Absorption, Distribution, Metabolism, and Elimination.

Okay, now let’s focus on elimination, which is the process of removing medications and their metabolites from the body through excretion.

This is done primarily by the kidneys, through glomerular filtration, meaning that certain medications can be filtered out as the blood flows through the glomerulus.

In addition, as the filtrate makes its way through the renal tubules, other medications from the peritubular capillaries get actively secreted into the tubular lumen, while others are passively reabsorbed into the blood.

Ultimately, the medications that are filtered, secreted, and not reabsorbed, end up being excreted from the body via urine.

Aside from the kidneys, medications can also be eliminated through the gastrointestinal or GI tract.

So orally administered medications that do not get absorbed along the GI tract are directly passed in feces.

On the other hand, some medications undergo a process of enterohepatic circulation, when a medication is transferred by the liver into the bile.

After that, bile carrying the medication is released into the GI tract, where the medication can be reabsorbed, entering the portal circulation, to ultimately return to the liver.

At this point, the process of enterohepatic circulation occurs again, and the liver transfers the medication back into the bile to be secreted into the GI tract, until it’s ultimately eliminated from the body through the feces.

Lastly, small amounts of certain medications can be cleared out of the body through the lungs as we exhale air, as well as through sweat, saliva, tears, and even breast milk.

Now, there are several factors that can affect the elimination of medications. First of all, impaired renal function or any condition that decreases the cardiac output, can result in decreased glomerular filtration rate, which can lead to the buildup of medications inside the body.

Urine pH also affects elimination. For example, if the urine is alkaline, acidic medications don’t get reabsorbed, so they get excreted in the urine instead.

Another factor affecting elimination is impaired hepatic function, which affects the liver’s ability to metabolize or break down the medication.

As a result, the elimination process can slow down and lead to increased levels of the medication inside the body for a longer period of time.

Finally, if certain medications are administered together, they can affect each other’s elimination. An example of this is probenecid, which is often given along with penicillin.

That’s because probenecid slows down the elimination of penicillin, letting it stay longer in the bloodstream, and increasing its effectiveness.

Now, let’s look at the half-life of a medication, which is the time required for the blood concentration of a medication to be reduced by half; and this mainly depends on the pharmacokinetic phases of metabolism and elimination.

So, for example, if a medication’s half-life is two hours, this means that it takes two hours for the concentration of that medication to drop from 100 to 50 mg/liter.

Alright, before administering any medication to your client, remember to keep in mind some of the general pharmacokinetic principles that relate to how the medication is eliminated from your client’s body.

Sources
  1. "Focus on Nursing Pharmacology" LWW (2019)
  2. "Pharmacology" Elsevier Health Sciences (2014)
  3. "Mosby's 2021 Nursing Drug Reference" Mosby (2020)
  4. "Saunders Comprehensive Review for the NCLEX-RN Examination" Saunders (2016)
  5. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)
  6. "Lehne's Pharmacology for Nursing Care" Saunders (2018)
  7. "Medication Safety Tips for the Breastfeeding Mom"  (2015)