What does the term "absorption" refer to in pharmacokinetics?

In pharmacokinetics, "absorption" specifically refers to the process by which a drug enters the bloodstream after administration. This is a critical phase that determines the onset of the drug's effects. During absorption, the drug must pass through biological barriers and often needs to be dissolved in bodily fluids to be properly absorbed. Factors such as the formulation of the drug, route of administration, and the characteristics of the drug itself can significantly influence how effectively and quickly a drug is absorbed into the systemic circulation.

Knowing how absorption works is essential for understanding overall drug bioavailability, which is the proportion of a drug that reaches the systemic circulation in an unchanged form. This phase is distinct from other processes in pharmacokinetics, such as elimination, which is concerned with how drugs are cleared from the body, distribution, which deals with how drugs disperse throughout body tissues, and metabolism, where drugs are chemically transformed into metabolites. Each of these processes plays a crucial role in the pharmacokinetics of a drug but focuses on different aspects of how drugs interact with the body.