Drug distribution in pharmacokinetics is governed by a molecule's polarity: hydrophilic (lipophobic, polar) drugs undergo aqueous diffusion and require pores or transporters to cross lipid membranes, while hydrophobic (lipophilic, nonpolar) drugs undergo lipid diffusion and cross membranes, including the blood-brain and blood-uterine barriers, without such aid. This polarity-based distinction determines central nervous system and fetal exposure, since only lipophilic drugs readily cross these barriers, though hydrophilic drugs can still passively permeate cells through pores rather than requiring active transport. All passive membrane transport, whether aqueous or lipid diffusion, is described by Fick's law, which relates flux to the diffusion constant, membrane surface area, membrane thickness, and concentration gradient.
Drug distribution in pharmacokinetics is governed by a molecule's polarity: hydrophilic (lipophobic, polar) drugs undergo aqueous diffusion and require pores or transporters to cross lipid membranes,…