Final answer:
The term used to describe what proportion of a drug reaches the systemic blood circulation and is available to reach the intended site of action is called bioavailability.
Step-by-step explanation:
The term used to describe what proportion of a drug reaches the systemic blood circulation and is available to reach the intended site of action is called bioavailability.
Bioavailability is an important pharmacokinetic parameter that determines the efficacy and predictability of a drug. It is influenced by factors such as administration route, drug formulation, and the extent of drug metabolism and excretion.
For example, when a drug is administered orally, it has to pass through the gastrointestinal tract and undergo first-pass metabolism in the liver before reaching the systemic circulation. This can significantly reduce the bioavailability of the drug compared to intravenous administration where the drug bypasses these barriers.