how do you know how many blood cells are in your body

Question: how do you know how many blood cells are in your body
- Keywords:
  blood cells,
  body
Asked by Oliver to Sam, Richard on 9 Mar 2016.
- Richard Sulston answered on 9 Mar 2016:
  
  Red blood cells carry oxygen around your body and are commonly counted to test peoples health. This is done by taking a small amount of blood from a vein, normally in your arm. The blood is looked at under a microscope where you can see the individual cells, they are then counted and by using this number you can do some maths to work out how many blood cells are in the body.
- Sam Smith answered on 10 Mar 2016:
  
  What Richard said! I don’t do a whole lot of blood analysis, so i’ll have to leave the answer to him on this one. Thanks for asking though, as I know the answer now!

Question: how do you know how many blood cells are in your body