How molecules cross membranes
Absorption is the trip from the site of administration into the systemic blood. Most drugs make this trip by passive diffusion — they simply dissolve through the fatty cell membrane, moving from where they are crowded to where they are sparse. To slip through fat, a molecule must be reasonably lipid-loving and uncharged. That last point matters because many drugs are weak acids or weak bases that flip between charged and uncharged forms depending on the surrounding pH.
The route changes everything
Where you put the drug decides how fast and how completely it is absorbed. The oral route is the most common — cheap, safe, convenient — but slow and incomplete. The intravenous route skips absorption altogether: the dose is placed directly in the blood, so 100% arrives, instantly. Other parenteral routes (under the skin, into muscle) sit in between. Each choice trades speed, completeness, convenience, and safety against one another.
Swallowed drugs face one extra toll. After leaving the gut, blood does not go straight to the body — it first drains through the liver, the body's chemical processing plant. The liver may destroy a large fraction of the dose before it ever reaches the rest of you. This pre-emptive loss is first-pass metabolism, and it is the main reason an oral dose is often much larger than the equivalent IV dose.
Bioavailability: the arrival score
Bioavailability, written F, is the fraction of an administered dose that reaches the systemic circulation intact. By definition an IV dose has F = 1 (100%), because nothing was lost on the way in. An oral dose is measured against that IV benchmark: if half is lost to incomplete absorption plus first-pass metabolism, F = 0.5. Bioavailability is the single cleanest summary of 'how much got in', folding absorption and the first-pass toll into one number.
Oral bioavailability (F)
dose actually in the blood (oral)
F = -----------------------------------------
dose actually in the blood (IV)
Example
Tablet swallowed: 100 mg
Reaches circulation: 35 mg (rest lost to
poor absorption +
first-pass liver)
F = 35 / 100 = 0.35 -> 35% bioavailable
Same drug given IV: F = 1.0 (100%) by definition.