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Pharmacokinetics vs. Pharmacodynamics

Pharmacokinetics vs. Pharmacodynamics
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  • 0:04 Pharmacotherapy
  • 0:33 Pharmacokinetics
  • 3:32 Pharmacodynamics
  • 6:23 Affective Factors
  • 6:52 Lesson Summary
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Lesson Transcript
Instructor: Lisa Cauthen

Lisa is a Registered Nurse with a 14 years of experience and a Masters Degree in Nursing Education. She has certifications in CPN, ACLS, PALS, and NRP.

Pharmacokinetics and pharmacodynamics are the studies of the interactions between the body and a drug. This lesson will explore the difference between the two and some key concepts within each one while using common medications as examples.

Pharmacotherapy

Pharmacotherapy, or using drugs for treatment, is based on two concepts: pharmacokinetics and pharmacodynamics. Most simply put, pharmacokinetics (PK) is how the body affects the drug while pharmacodynamics (PD) is how the drug affects the body.

Let's look deeper into these concepts and, when they get confusing, explore examples to try to make them applicable to your everyday life.

Pharmacokinetics

Pharmacokinetics is derived from ancient Greek. Pharmakon means 'drug' and kinetikos means 'moving or putting in motion.' Pharmacokinetics describes how the body moves and affects a specific chemical or drug so that the drug can have a pharmacodynamic response. Pharmacokinetics consists of four components: absorption, distribution, metabolism, and excretion.

We'll first take a look at absorption. Absorption is the process of a drug entering the blood circulation. Absorption depends on the administration route and can either be enteral (by in the GI tract, such as by mouth, feeding tube, or rectal suppository) or parenteral (not in the GI tract, such as an injection or topical medication). Additional factors that can affect the amount of the drug absorbed include the drug's formulation (extended release versus immediate release), blood flow to the area of absorption, and GI motility (for enteral medications).

Let's take a look at some examples. Common enteral medications include acetaminophen and ibuprofen. Common parenteral medications include insulin and heparin. Some medications, such as diphenhydramine and penicillin, have both enteral and parenteral forms.

Now let's take a look at distribution. Distribution is the drug's dispersion through the body's fluids and tissues as it travels to its site of action. This is dependent on blood flow, both to the area where the drug was administered and throughout the body as well as protein binding. If a drug binds to a protein it becomes stuck to it and cannot exert its effect on the body. It's an inverse proportion: the more a drug binds to protein, there is less of it available for distribution.

Now let's take a look at metabolism. Metabolism, or biotransformation, is recognition by the body that the drug is present and the transformation of the drug into usable parts. Most drugs are metabolized in the liver via the cytochrome P450 family of enzymes, but other sites may include the kidneys and intestines.

Let's look at an example. Anti-cholesterol drugs known as statins and grapefruit juice are metabolized in using the same enzyme in the liver and will fight each other for it. That is why people taking statins should avoid taking them with grapefruit juice.

Finally, let's take a look at excretion. Excretion, also known as elimination, is removal of the drug from the body. The primary site of excretion is the kidneys. Others include the lungs, bile, saliva, sweat glands, and skin. Half life is the amount of time it takes for half the drug to be eliminated from the body. After three to five half-lives the drug is considered undetectable and unable to exert a pharmacodynamic effect.

Pharmacodynamics

The term pharmacodynamics comes from the ancient Greek word dynamikós, which means 'force or power.' Pharmacodynamics is the force, or power, the drug has on the body. Most drugs bind to specific receptor sites within the body to exert their force.

Pharmacodynamics concepts include affinity, efficacy, and potency, and whether the drug is an agonist or antagonist. Pharmacodynamics includes both the desired effect of the drug as well as the undesired, or side, effects.

Let's take a look at affinity and efficacy. Affinity is the attraction between a drug and its receptor. It's important to know that there are many different receptors in the body and not every drug will attach to every receptor. Efficacy is the drug's ability to activate the receptor once it has bound to it. Some drugs have both a high affinity and high efficacy, while others have high one and low the other. Together, affinity and efficacy affect potency, or the amount of a drug that's necessary to produce a desired effect. The greater the affinity and efficacy, the greater the potency.

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