biology please help?

Question

when a patient recieves an interavenous solution it usually is isotonic with reference to blood cells. this means it has the same concentration of dissolved materials as blood. please explain why this is neccessary?

Answer 1

The above poster is wrong in regards to the membrane of RBC's. It is not not semi-permeable. If it was semi-permiable, then that would mean anything small enough to pass through the membrane will pass through it. The membrane is "semi-differential". Meaning, that through osmosis, only certain gases, mostly oxygen, water, and a few simple lipid, protein and sugar subtances can pass through the membrane. But nothing else, through regular osmosis.

The most commonly used isotoni IV solution is normal saline, a solution of sodium chloride at 0.9% concentration, which is close to the concentration of red blood cells. The dissolved materails sodium chloride are ions, and won't diffuse into red blood cells through normal osmosis. The rest of what the above poster said was correct. Hypotonic solutions will swell RBCs, and Hypertonic solutions will crenate them, but mostly because of the water content, not the dissolved material content.

Answer 2

Because the membranes of red blood cells (and all cells) are permeable to water, the concentration of solute in a solution affects the movement of water into or out of the cells. If the IV solution was hypotonic to RBCs, water would move into the cells and they would burst. If the IV solution was hypertonic to RBCs, water would leave the RBCs and the cells would undergo crenation. Both of these is bad for the red blood cells.