How do they check for the amount of plasma contained by blood?

Isn't plasma the blood without it's components of cells and plates ?
Answers:
They can determine the relative amount of plasma in your blood by taking a token of your blood and centrifuging it out (spins very fast, larger molecules fall to the bottom). Plasma sits at the top of the mix. Plasma is the fluid and water-soluble unbound nutrients (and ions) within the blood.

They can determine the total amount of blood in your system by introducing a drug, tagged radioactively, which only travels within your bloodstream. By measuring concentration (radiation) changes over time they figure out your blood volume (and beside that, total amount of plasma).

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