3.8 : 药物吸收机制：旁细胞转运、跨细胞转运和囊泡转运

After administration, drugs must cross cell membranes to reach their target site in the body. For example, oral drugs must navigate intestinal epithelial membrane barriers to enter the systemic circulation.

Drugs with a molecular weight lower than 500 Daltons bypass cell membranes and take the paracellular route. They diffuse through the gaps between neighboring cells.

Most drugs prefer the transcellular route to directly traverse through the cell membranes via two distinct mechanisms.

Passive transport allows drugs to pass freely through the lipid bilayer along the concentration gradient.

Active transport, on the other hand, utilizes membrane transporters, and ATP to move drugs against the concentration gradient. In some cases, drugs are exchanged for other solutes which are moving along their concentration gradient.

Lastly, vesicular transport involves the cell membrane invagination to internalize large macromolecules, insoluble or soluble particles inside the cell. Engulfment of macromolecules or large foreign particles is termed phagocytosis, whereas internalization of fluids containing small particles is pinocytosis.