It is important to remember that the body is made up of several theoretical fluid compartments (extracellular, intracellular, plasma, etc.), and Vd attempts to describe the fictitious homogenous volume in a theoretical compartment. It is defined as the amount of drug in the body divided by the plasma drug concentration. This metric is a common method of describing the dissemination of a drug. To be effective, a medication must reach its designated compartmental destination, described by the volume of distribution, and not be protein-bound in order to be active. This is the concentration of the drug at its designed receptor site. The goal of the distribution is to achieve what is known as the effective drug concentration. These factors may be influenced by the polarity, size, or binding abilities of the drug, the fluid status of the patient (hydration and protein concentrations), or the body habitus of the individual. In its simplest sense, the distribution may be influenced by two main factors: diffusion and convection. This varies based on the biochemical properties of the drug as well as the physiology of the individual taking that medication. By calculating the integral of that curve, bioavailability can be expressed as a percentage of the 100% bioavailability of intravenous administration.ĭistribution describes how a substance is spread throughout the body. The AUC is a method of calculating the drug bioavailability of substances with different dissemination characteristics, and this observes the plasma concentration over a given time. Other modes of administration may delay certain quantities of drugs to arrive in circulation at the same time (intramuscular, oral, transdermal), giving rise to the use of the area under the plasma concentration curve (AUC). These processes will be discussed in greater detail under metabolism.
Once absorbed by gut transporters, the medications then often have to undergo "first-pass metabolism." When oral medication is administered, it is often processed in large quantities by the liver, gut wall, or digestive enzymes, subsequently lowering the amount of medication that arrives in circulation therefore, having a lower bioavailability. The digestive enzymes begin the process of metabolism for oral medications, already diminishing the amount of drug arriving in circulation before being taken up. Oral medications, once swallowed, must navigate the acidity of the stomach and be taken up by the digestive tract. This concept is especially important in orally administered medications. This makes intravenous administration the gold standard regarding bioavailability. For example, when administering medication intravenously, 100% of the drug arrives in circulation virtually instantly, giving this method a bioavailability of 100%. It can be a direct reflection of medication absorption. īioavailability is the fraction of the originally administered drug that arrives in systemic circulation and depends on the properties of the substance and the mode of administration. Once in the stomach, the low pH may begin to chemically react with these drugs before they even arrive in the systemic circulation. For instance, an oral medication may be delayed in the throat or esophagus for hours after being taken, delaying the onset of effects or even causing mucosal damage. This is especially important in the case of oral medications. The process of absorption also often includes liberation, or the process by which the drug is released from its pharmaceutical dosage form. Each of these methods has its own absorption characteristics, advantages, and disadvantages. There are many possible methods of drug administration, including but not limited to oral, intravenous, intramuscular, intrathecal, subcutaneous, buccal, rectal, vaginal, ocular, otic, inhaled, nebulized, and transdermal. Absorption affects the speed and concentration at which a drug may arrive at its desired location of effect, e.g., plasma. Absorption is the process that brings a drug from the administration, e.g., tablet, capsule, into the systemic circulation.
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