Orally administered drugs must pass through the liver (via the hepatic portal vein) prior to reaching general circulation and site of action. This is referred to as

a. enterohepatic circulation.
b. first-pass metabolism.
c. passive diffusion.
d. distribution.

b. Drugs are absorbed from the gut but may be biotransformed by enzymes in the gut and liver before reaching the systemic circulation. This process lowers their degree of bioavailability.