Pharmacology Basics, part 3: Pharmacokinetics: Absorption, Distribution, Metabolism, Excretion
by Cathy Parkes August 09, 2021 Updated: March 31, 2022 7 min read
In this article, we'll explain pharmacokinetics—how drugs are absorbed, distributed, metabolized, and excreted by the body. You'll learn how a full stomach, grapefruit juice, your genetics and your age might change a drug's effectiveness in your body.
This series follows along with our Pharmacology Basics and Safe Medication Administration Flashcards for Nursing Students which are intended to help RN and PN nursing students build a strong foundation going into Pharmacology and as preparation for the ATI, HESI, and NCLEX.
What is pharmacokinetics?
Pharmacokinetics is the study of movement and processing of drugs within the body.
Pharmaco- means having to do with drugs or medicine, and -kinetics is movement. Check out our Medical Terminology and Abbreviations Flashcards for more word part hints that make understanding medical terms easy!
In pharmacokinetics, absorption is how a medication gets from the site of administration into the bloodstream.
What affects absorption?
Pharmacokinetic absorption can be affected by the route of administration, medication solubility, dose, or formulation, the surface area of the absorption site, presence of food in the stomach, and the pH of the stomach acid.
Route of administration
The delivery route of a medication affects absorption. A medication administered via IV goes straight into the bloodstream, so absorption is immediate. However, an oral medication has to go through the GI tract, so absorption is a little slower.
Medication solubility, dose, formulation
The solubility, dose, and formulation of the medication affect absorption. For example, a liquid medication will be absorbed quicker than a tablet.
Surface area of absorption site
The surface area of the absorption site affects drug absorption. For example, some drugs are absorbed readily in the intestines due to the relatively large total available surface area.
Presence of food in the stomach
The presence of food in the stomach affects absorption. Oral medications can be absorbed through the lining of the stomach, and if food is in there taking up space, it will reduce the absorption. Sometimes this is desired in order to regulate the speed and amount of absorption of the drug.
Stomach acid pH
The pH of the stomach acid affects absorption. As we age, our pH increases (becomes more alkaline) and that decreases absorption of a medication.
In pharmacokinetics, distribution is how a medication gets from the bloodstream to the site of action.
What affects distribution?
Pharmacokinetic distribution is also affected plasma protein binding, permeability at the destination site, and circulation.
Plasma protein binding
Plasma protein binding is an important factor affecting distribution. Medications need protein (e.g., albumin) to get to the site of action. If a patient has low levels of albumin, this will negatively affect protein binding.
Permeability at the destination site
Permeability at the destination site affects absorption. For example, non-lipid-soluble medications will be unable to penetrate the blood-brain barrier, which affects distribution.
Circulation (blood flow) is another key factor that affects distribution. For example, if a patient has peripheral arterial disease, which means that they have trouble with blood flow down to the extremities, that will affect the distribution of medications to their extremities because the blood flow to the distribution site is interrupted.
Peripheral arterial disease is covered in the cardiovascular section of our Medical-Surgical Nursing Flashcards.
In pharmacokinetics, metabolism is the inactivation of medications by enzymes. This occurs primarily in the liver, but other organs that may be involved include the GI tract, the lungs, and the kidneys.
If a drug "has been metabolized" it has been rendered inactive by the process of metabolism.
What affects metabolism?
Metabolism can be affected by liver function, genetics, sex, hormones, environmental factors, and diet.
The most important factor influencing drug metabolism is liver function, because metabolism primarily occurs in the liver. Infants, elderly patients, and patients with impaired liver function (e.g., with cirrhosis) will have impaired metabolism.
The first-pass effect
Certain medications have a first-pass effect, which means as they first pass through the liver they become inactivated and must be administered through a parenteral route (IV, subcutaneous, or intramuscular).
Metabolism can be affected by genetics. Some of the specific drug-metabolizing enzymes have been found to be genetically more present in some people than in others. If you have more of these enzymes, you will metabolize the drug faster, and vice versa.
Sex & hormones
Drug metabolism can be affected by the sex of a patient. The prevalence of some drug-metabolizing enzymes have been found to be sex-dependent.
Hormones, like those found in oral contraceptives, have also been found to affect drug metabolism.
Environmental factors can affect drug metabolism. Some chemicals in the environment are thought to impact the effectiveness of drug-metabolizing enzymes.
Diet is known to affect drug metabolism because the specific chemicals in certain foods may change the effectiveness of the body's drug-metabolizing enzymes.
That's why certain foods, like grapefruit juice, are contraindicated for some medications (e.g., colchicine). When grapefruit juice inhibits drug-metabolizing enzymes, it could lead to too much of the drug in the body and thus, toxicity.
In pharmacokinetics, excretion is the elimination of medications from the body. This primarily takes place in the kidneys via urine, but other routes include the saliva, sweat, tears, and feces.
What affects excretion?
Pharmacokinetic excretion can be affected by kidney or liver dysfunction, urinary or medication pH, and age.
Kidney or liver dysfunction
Because the kidneys and liver are primarily responsible for excretion, kidney or liver dysfunction can result in less excretion, and therefore, potential drug buildup and toxicity.
Urinary and medication pH can also affect excretion. If a drug is acidic, it is best excreted in alkaline urine, and if a drug is alkaline, it is best excreted in acidic urine.
Age is another key factor affecting excretion of drugs. Newborns and older patients have decreased kidney and liver function which impairs (slows) their excretion of medications and increases their risk for toxicity. If medications cannot be excreted, they may build up in the body too long which could cause toxicity.
Hi, I'm Cathy with Level Up RN. If you are new to our channel, I invite you to subscribe to our channel. We have hundreds of free videos to help you along your way to becoming a nurse. In this particular video, we are going to talk about pharmacokinetics, and specifically, we'll be talking about the four phases of pharmacokinetics, so absorption, distribution, metabolism, and excretion. At the end of the video, I'm going to give you guys a little quiz to test your understanding of the information that I'm going to provide in this video, so definitely stay tuned for that.
All right. Let's first talk about absorption. Absorption is how a medication gets from the site of administration into the bloodstream. So if we're talking about an IV medication, that's going straight in the bloodstream, so absorption is going to be immediate. However, if we're talking about an oral medication, that has to first go through the GI tract, so absorption is going to be a little slower.
Other things that affect absorption include the solubility of the medication as well as the dose and formulation. So for example, a liquid medication will be absorbed quicker than a tablet. Other things could include the surface area of the absorption site as well as the presence of food in the stomach and the pH of the stomach acid. So as we get older, our pH increases, becomes more alkaline, and that really decreases absorption of a medication. So as we get older, absorption rate decreases.
After absorption, we have distribution. Distribution is how a medication gets from the bloodstream to the site of action.
And distribution is also affected by multiple factors. One factor is plasma protein binding. So medications need protein such as albumin to get to the site of action. So if a patient has low levels of albumin, this will negatively affect protein binding.
Then we have permeability at the destination site. So for example, non-lipid-soluble medications will be unable to penetrate the blood-brain barrier, so distribution is definitely affected.
Circulation is another key factor that affects distribution. So when I say circulation, I mean blood flow. So for example, if a patient has peripheral arterial disease, which means that they have trouble with blood flow down to the extremities, that will affect distribution of medications from getting to their extremities because that blood flow is interrupted.
After distribution, we have metabolism, which is the inactivation of medications by enzymes. This occurs primarily in the liver, but other organs that may be involved include the GI tract, the lungs, and the kidneys.
So in terms of what influences metabolism, the most important factor is really liver function because metabolism primarily occurs in the liver. So infants, elderly patients, and patients with impaired liver function such as cirrhosis will have impaired metabolism.
We also have what's called a first-pass effect with certain medications. That means that these medications, as they have their first pass through the liver, become inactivated and must be administered through a parenteral route.
So when I say parenteral, I mean a route other than the GI tract. So this could include an IV route, subcutaneous injections, or intramuscular injections.
And then metabolism can also be affected by genetics, by sex, by hormones, as well as environmental factors and diet.
Okay. After metabolism, we have excretion, which is the fourth phase of pharmacokinetics. With excretion, this is how we eliminate medications from the body. This primarily takes place in the kidneys via urine, but other routes include the saliva, sweat, tears, and feces.
In terms of what affects excretion, we have kidney or liver dysfunction, which would definitely impair excretion.
Urinary and medication pH can also affect excretion.
And then age is another key factor. So newborns and older patients have decreased kidney and liver function; therefore, that impairs their excretion of medications and increases their risk for toxicity.
Okay, quiz time. I have three questions for you. The first question is a true-false question. Older adults have a higher gastrointestinal pH which impairs absorption. True or false? The answer is true. Second question. How a medication gets from the bloodstream to the site of action describes what phase of pharmacokinetics? If you said distribution, you are correct. Third question. What is the primary organ responsible for metabolism in the body? The answer is the liver. So I hope you did well on that quiz. If not, feel free to watch that video again or review the information in our flashcards. Thank you so much for watching.
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