Why do hospitals inject saline into your bloodstream instead of just water?
In: 110
One of the most important thing about infusion is that it has to be isotonic or iso-osmotic (that’s the same thing).
It means that the osmotic pressure of blood and the infusion is the same.
#What is osmotic pressure?
Osmotic pressure is basically the amount of solved things in the water. A very pure water can solve a lot of stuff and it has low osmotic pressure. If it is already solved stuff in it, you cannot add too much more and it’s a high osmotic pressure.
If you have a membrane between clean water and a solution, they will try to equalize. It means that water goes through the membrane from the pure-water side to the solution, and salts go from the solution side to the pure water. It stops when there’s a balance. This process is osmosis and it goes until the osmotic pressure on both sides are the same. Meaning that the solvent concentration is the same.
Osmotic pressure does not mean that the two sides are exactly the same. You can have two different materials solved in the two sides and have the same osmotic pressure, as long as the concentrations are in balance.
#What’s in the blood?
The blood has cells in it and liquid part called plasma. The cells are little living things and they are surrounded by a membrane. The cells have a certain osmotic pressure, so the liquid part must have the same osmotic pressure. Otherwise the cells would gain water through the membrane and blow up, or loose water and shrink.
So if you give pure water as an infusion, you mess up the osmotic pressure and blow up the cells. That’s bad. That’s why you must give something in the water so it’s iso-osmotic.
#Why salt?
Salt is the simplest thing that you can have. Blood already contains 140 units of sodium and 100 units of chloride, total 240 units. (Units are millimole per liter.) The physiological saline has about 150 units of both (total 300). So as you see, it’s considerably more salt than the blood has, but the reason is that the blood has a lot more things: potassium, magnesium, proteins etc. The surplus salt in the infusion is to balance the osmotic pressure of the non-salt stuff in the blood. So the final osmotic pressure is the same.
There are other infusions that contain sugar or lactate or something else. They have less salt so the final osmotic pressure is always the same as the blood.
Latest Answers