Pharmacology, part 5: Pharmacokinetics

Hi. I'm Cathy with Level Up RN. In this video, I will be discussing pharmacokinetics, and at the end of the video, I'm going to give you guys a little quiz to test your knowledge of some of the key points I'll be covering, so definitely stay tuned for that. Specifically, we will be talking about absorption, distribution, metabolism, and excretion. So we'll be discussing how a medication enters the body, how it moves about the body, how it's changed in the body, and how it leaves the body. And as always, I will be following along with our Level Up RN pharmacology flashcards, so if you have our flashcards, definitely pull them out, so you can follow along with me.

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 can 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, nonlipid 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.