Pharmacokinetics refers to the movement and modification of a drug or medication inside the body. Once a medication is administered, it’s first 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. This process can be broken down into four components with the acronym ADME; which stands for Absorption, Distribution, Metabolism, and Elimination.
Now, the first step of pharmacokinetics is absorption, which refers to the process of how a medication goes from the site of administration to the bloodstream. To do so, the medication may need to cross one or more cell membranes; and this can occur via passive transport, which requires no energy, or active transport, which requires energy in the form of ATP.
With passive transport, the medication goes down its concentration gradient, so from an area of higher concentration to an area of lower concentration. There are two types of passive transport: passive diffusion and facilitated diffusion. Passive diffusion allows small, lipid-soluble, and nonpolar medications to freely move across the membrane. Facilitated diffusion, on the other hand, allows larger, water-soluble, and polar medications to move across the membrane through transport proteins like channels and carrier proteins.
Now, some medications need to be transported through active transport against their concentration gradient, so from an area of lower concentration to an area of higher concentration. This can be achieved with specific carrier proteins that use ATP as an energy source to pump medications against their gradient. Now, the rate of the absorption, or how quickly this process occurs, as well as the extent of the absorption, or how much of that medication reaches the bloodstream, can be affected by several factors. One of them is the pH of the environment where absorption takes place.
Okay, so most medications are either weak acids or weak bases; weakly acidic medications will be better absorbed in an acidic environment, like the proximal duodenum, while weakly basic medications are more likely to get absorbed in an alkaline environment, like the distal ileum of the small intestine. Note that even though the stomach is acidic, it’s not suitable for the absorption of even weak acids, mainly because of its thick mucus layer.
Okay, now another factor that affects absorption of a medication is the surface area available. In this regard, a good place for absorption is the small intestine, which has a very large total surface area, approximately the size of a tennis court! Other factors that can affect the rate and extent of absorption include the amount of blood supply to the absorption site, as well as the presence of other substances, such as certain foods in the gastrointestinal tract, that can either promote or inhibit absorption.
Besides these, the route of administration and pharmaceutical preparation of a medication also have some role in its absorption. For example, a medication given sublingually has a higher rate and extent of absorption than its oral form, as it bypasses the stomach acids.
Similarly, pharmaceutical preparations like enteric coatings are made in such a way that, when taken orally, they protect the medication from stomach acids; only after reaching the small intestine, the coating gets dissolved, allowing the medication to be absorbed. A practical measure of absorption is bioavailability, which refers to the proportion of the medication that actually reaches the bloodstream. For example, if a client takes 100 mg of aspirin orally, and only 60 mg of this is absorbed into the circulation, the bioavailability is 0.6 or 60%.