It’s Why You Put Saline Solution in an IV [...]

When people get dehydrated, or when they need an intravenous drug they are given an IV with a saline solution.

Why is this?

Well, consider that your blood is pretty salty, and the blood cells in the solution that is your blood are used to that salinity. Think a minute what would happen to the blood cells if suddenly they were surrounded by a hypotonic solution (distilled water) rather than a hypertonic one (saline or blood).

What would happen to the blood cells? Would they expand, contract, or stay the same?

They’d expand. Let’s walk through this.

Your blood cells currently exist in osmotic balance with your blood. Your blood is made less salty via the addition of water.

Now the saltiness has to equalize, but it can’t equalize by virtue of moving the solute (in this case salt) because the solute cannot move through cell membranes. So the water has to move. And since the cells have more solute, the water will move into the cells.

Eventually, the cells will burst.

The opposite is true if the saline solution is too salty. In this case the cells will shrivel.

Don’t believe me? Take a look under a microscope: