system
Given the following post, respond as if you're commenting on Reddit:
human
Why do medical professionals use Saline Solution as opposed to Distilled Water?

Saline seems to be the most universally used liquid in medicine. I've seen it used in IV drips, to clean out wounds, and to clean needles between stitches. Why is Saline used as opposed to simple water? Is it more sterile? Easier to make/cheaper? When did it become commonly used?
gpt
Saline solution is used is to maintain a balance of osmotic pressure between the body's cells and fluids.  Cell membranes are selectively permeable which, in this case, means that water can enter and leave the cell freely but most species in solution cannot.  Two systems separated by a selectively permeable membrane like this will seek an equilibrium, with water moving however it has to in order to make both systems roughly equally concentrated.

Using pure distilled water (hypotonic) would cause water to rush into the cells to dilute the contents and reach equilibrium, resulting in popped cells.  Using more concentrated solutions (hypertonic) would draw water out from the cells and result in them being dessicated.