Question

when a doctor injects a fluid into a persons bloodstream, he always uses a fluid that has the same concentration as the blood. why?

Answer 1

If a more concentrated solution were added to a person's blood, then water molecules in intesitial fluid would flow down their concentration gradient into the blood. It would increase the total amount of fluid in the blood vessels.

Simiarly, if a solution with more water than blood were added to the bloodstream, water molecules would flow down their conentration gradient from the blood to the body tissue and cause edema.

There are possibly other (and better) reasons, but that is the one that first comes to my mind.

Answer 2

what about osmotic pressure

Answer 3

That basically is osmotic pressure. Movement of water molecules down a concentration gradient. I should have made that clearer. ;D