What happens to drugs absorbed from the GI tract before they enter systemic circulation?

Drugs absorbed from the gastrointestinal (GI) tract typically enter the portal circulation, which directs them to the liver before they reach systemic circulation. This process is referred to as the first-pass effect or first-pass metabolism. The portal vein transports these drugs straight to the liver, where they can be metabolized by hepatic enzymes.

This is significant because it means that the concentration of the drug that ultimately enters the systemic circulation can be substantially reduced if the liver metabolizes it extensively. This is particularly important for oral medications, as their bioavailability can be significantly affected by this first-pass metabolism.

The other choices do not accurately describe the path that drugs take after being absorbed from the GI tract. Drugs are not rapidly metabolized by blood plasma; instead, they are absorbed and then pass to the liver. They are also not filtered through the kidneys before entering the bloodstream, as that occurs later in the pharmacokinetic process. Finally, drugs are not eliminated before reaching the bloodstream; rather, they are absorbed and then transported to the liver via the portal vein. Understanding this pathway is crucial for optimizing drug formulations and understanding their therapeutic profiles.