absorption

Absorption is the journey of a drug from where you put it — the gut, the skin, a muscle — into the bloodstream that carries it around the body. Picture a swallowed tablet: the drug must first dissolve, then slip across the lining of the intestine into the capillaries underneath. Until that crossing happens, the drug is technically still 'outside' the body.

Mechanistically, most small-molecule drugs cross the gut wall by passive diffusion: lipophilic, un-ionized molecules dissolve through the lipid membrane down a concentration gradient, a process described by Fick's law and shaped by the drug's pKa and the local pH. Some drugs instead ride carrier proteins (active or facilitated transport), and a few squeeze between cells (paracellular) or are taken up by endocytosis. For oral drugs, dissolution in gut fluid and permeability through the membrane are the two rate-governing steps the BCS captures.

Absorption is not the same as bioavailability. A drug may be fully absorbed across the gut wall yet still be heavily metabolized in the gut wall and liver before reaching the general circulation — the first-pass effect. So 'amount absorbed' and 'amount that becomes systemically available' can differ, and careful writing keeps the two apart.