What Is Pharmacokinetics? - Definition & Principles

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  • 0:00 Definition of Pharmacokinetics
  • 0:30 Absorption & Distribution
  • 2:10 Metabolism & Excretion
  • 3:07 Other Concepts of…
  • 3:58 Lesson Summary
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Lesson Transcript
Instructor: Heather Zonts

Heather has taught in AD and BSN Nursing programs and has a master's degree in nursing.

Ever wonder what happens when medications enter into the body? This lesson will discuss the basics of pharmacokinetics from the time the medication enters the body until the point of excretion.

Definition of Pharmacokinetics

Pharmacokinetics refers to what happens to a medication from entrance into the body until the exit of all traces. Four processes encompass the pharmacokinetics of a medication. They are absorption, distribution, metabolism, and excretion. Each of these processes is influenced by the route of administration and the functioning of body organs.

Let's look at these processes in further detail.

Absorption and Distribution

Absorption refers to the movement of a medication from where it entered the body to the bloodstream. Enteral medications are typically absorbed through the stomach or intestinal mucosa. These include any medication that is taken by mouth. Factors that can influence absorption of enteral medications are food in the stomach, drug solubility, and blood flow.

Another route of absorption is the parenteral route, which typically refers to medications that are injected. The rate of absorption will vary depending on the type of injection. Intravenous medications are associated with this category. They are injected directly into the bloodstream, and therefore are absorbed the fastest. Additionally, intramuscular medications, or those where medication is injected deep into the muscles, are absorbed more rapidly than subcutaneous injections, where medication is injected into the tissue between the skin and muscle, due to the increased blood supply provided to the muscles.

After absorption comes distribution, or transport of the medication via the bloodstream to the intended action site. The blood supply to the area will determine the rate of distribution. For example, areas such as the heart and brain have an increased blood supply, so medication will act more rapidly in these areas.

Medication is also distributed to areas of metabolism and excretion since once a medication enters the bloodstream it's distributed throughout the body. Factors that influence distribution include, but are not limited to, the chemical consistency of the medication, the amount of medication, potential drug-drug interactions, local blood flow, and membrane permeability.

Metabolism and Excretion

Metabolism refers to the breakdown of medication into an inactive form. Metabolism is also referred to as biotransformation because of the effect on the chemical consistency of medication. The primary organ of medication metabolism is the liver, although some medications are metabolized in the kidneys, lungs, and intestinal tract, to name a few. Factors that influence the metabolism of a medication are the functioning capacity of the metabolizing organ and an individual's overall health status.

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