Lessons · Medicine foundations · bioavailability as a fraction
Bioavailability: the fraction that actually gets in
Bioavailability F is the share of an oral amount that reaches the bloodstream; the amount absorbed is the amount taken times F, and the oral amount matching an intravenous one is the intravenous amount divided by F.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A tutorial asks why the figure by mouth is larger than the figure into a vein for the same substance, and the answer is one fraction. Which way to use it is the part that goes wrong: one direction multiplies and the other divides, and they differ by a lot when F is small.
How to think about it
Say the sentence before you write the sum. Going from swallowed to absorbed, less gets in, so multiply by a fraction smaller than one. Going from what you want in the blood back to what has to be swallowed, more is needed, so divide. Check the direction against the size of the answer.
Worked example
F is given as 0.5, so half of what is swallowed reaches the bloodA fraction of one, with no unit.
A 300 mg oral amount: 300 × 0.5 = 150 mg absorbedSwallowed to absorbed. Less, so multiply.
To match 200 mg given into a vein: 200 / 0.5 = 400 mg by mouthAbsorbed to swallowed. More, so divide.
Your turn
F is 0.6 and the oral amount is 450 mg. Write the line that gives the amount absorbed.
450 × 0.6 = mg
Solve one, graded on the server
The trap
Multiplying when the question asked which oral amount matches an intravenous one. With F of 0.5 the two answers are 100 mg and 400 mg from the same starting figure, and only the sentence you said before writing tells you which one the question wanted.