Lessons · Medicine foundations · blood volume from body weight
Blood volume: a set number of millilitres for every kilogram
Circulating blood volume is estimated as a given number of millilitres per kilogram of body weight, so working it out is one multiplication.
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 question asks what share of the circulating volume a 450 mL donation is. The top of that fraction is handed to you; the bottom is the number you have to make, and it comes from the weight and a figure the question gives you in millilitres per kilogram.
How to think about it
Read the millilitres per kilogram out of the question rather than from memory, because a problem about an adult and a problem about a newborn will give you different ones. Multiply by the weight in kilograms. Divide by 1000 if the answer wants litres.
Worked example
The problem gives 70 mL per kilogramPart of the question. A different problem may give a different figure and that one wins.
A 70 kg adult: 70 × 70 = 4900 mLWeight in kilograms times millilitres per kilogram.
4900 / 1000 = 4.9 LThe same volume in litres.
A 450 mL donation: 450 / 4900 = 0.092, which is 9.2 %A share of the whole, once the whole is known.
Your turn
The problem gives 70 mL per kilogram and a weight of 60 kg. Write the line that gives the blood volume in millilitres.
60 × 70 = mL
Solve one, graded on the server
The trap
Using a figure you remember instead of the one printed in the question. Every figure on this track comes with its problem, and the one in the problem is the one being marked.