In pharmacokinetics, two major concentration gradients are between which two sites?

• A. Site of Administration and Site of Action
• B. Blood and Tissue
• C. Plasma and Interstitial Fluid
• D. Gastrointestinal Lumen and Blood

Answer: (A)

In pharmacokinetics, the movement of a drug is driven by diffusion along a concentration gradient from where the drug is placed into the body to where it exerts its effect. The key gradient is from the site of administration (where the drug is initially present at high concentration) to the site of action (where the drug needs to reach a sufficient concentration to produce a response). This single gradient governs the overall path of absorption into systemic circulation, distribution to tissues, and ultimately the pharmacologic effect.

Movement between blood and tissue, or between plasma and interstitial fluid, are important steps within that journey, but they are downstream consequences of the primary administration-to-action gradient. The gradient from the gastrointestinal lumen to the blood is specifically about absorption for oral dosing, not the entire PK process.