In the simplified Fick's law, the constant P represents what combination of factors?

• A. The product of diffusion coefficient, surface area, and partition coefficient divided by tissue thickness
• B. Blood flow
• C. Absorption site pH
• D. Temperature

Answer: (A)

The permeability constant reflects how readily a drug can cross a barrier, combining the factors that drive diffusion and the geometry of the barrier. It scales with how fast the molecule diffuses in the barrier (diffusion coefficient), how much surface area is available for exchange (surface area), and how well the drug partitions into the barrier from water (partition coefficient). It is inversely related to the thickness of the barrier (thinner barriers offer less resistance). Putting these together gives P proportional to D × A × K divided by h. That’s why the description—product of diffusion coefficient, surface area, and partition coefficient, divided by tissue thickness—best matches the permeability constant.

The other considerations—blood flow, absorption site pH, and temperature—affect delivery and conditions around diffusion but are not part of the intrinsic permeability constant in this simplified formulation.