Why do hospitals inject saline into your bloodstream instead of just water?
In: 110
Basically your blood is a lot more like saline than pure water. Giving you just water would cause all sorts of electrolyte imbalances. Saline doesn’t do this but still provides adequate hydration (hope this is a good explanation, I’m new at answering these)
Source: I work at a hospital
Your blood is not just water. It is a mix of water, salt, and of course many other things. But at the very basic level, the right balance of water and salt are extremely important to your body functioning. Salt and water want to mix and will push across certain boundaries in your body to accomplish this. Too much water will try to force its way into your cells to mix with the salt in your cells. Too much salt will suck water *out* of your cells to mix with the salt in your bloodstream. Some cells like plant cells and your own kidney cells are designed to compensate for an incorrect balance of salt and water and stop the movement of water that would occur as a result of this. But the rest of the cells in your body are not prepared to handle this – they expect your blood to have the proper salt levels and if this is wrong there are bad effects. The hospital isn’t “making your blood salty”, they are keeping it how it’s supposed to be
Hospital-based doctor here. Sometimes we do give “water”. It’s just usually got sugar in it, too (e.g., D5W contains dextrose, a sugar, which your body breaks down, leaving only water). Water infusions leave the blood and goes into tissues. It’s good for overall hydration but bad for keeping things like the blood pressure up, because it leaves the blood stream. It also has (wanted or not) an effect of diluting the natural amount of salt in your blood. If that salt level drops too low it can cause seizures and other brain problems. Salt water (saline) stays in the blood stream longer, so it can help with things like blood pressure. Even with salt water we still say only 1/3 of it stays in the blood stream. Benefit is it doesn’t dilute salt in the blood.
Basically, think of the liquids in your body like a soup. If you add extra water, but no seasonings (like salt) the soup is gonna get watered down and flavorless
Throw in the fact that “flavourless” in this example means “your heart stops working because it needs sodium to beat”, and suddenly that becomes very important to avoid.
They don’t wanna rinse out all your electrolytes when you go to pee, and dehydration tends to make eating problematic. So they use saline to keep those salt levels relatively stable while they get water in you.
Humans came from the sea. We carry the salty ocean water with us. So any fluid you put in your body should not change the salt level too much. Modern oceans have become saltier over time due to run-off. We only think of ourselves as pure land animals but we can only be on the land because we carry our ocean inside of us and we must constantly replenish the ocean in us.
The saline they inject is the same saltiness-level as your blood (the fancy medicine word is “isotonic”). If they injected just water, it would be less salty than your blood and therefore dilute your blood salt levels. Your blood is salty for a reason, so messing up the salt levels by adding pure water is (usually) bad. By adding saline instead, they can add fluid volume without changing the existing salt level.
Saline isn’t perfect, Ringers solution is a much match to blood electrolytes but more expensive.
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.
Water would dilute the sodium content of your blood putting you at risk for hyponatremia which is a dangerous condition.
This is why sports drinks are relatively high in sodium, you want to replace the salt you’re sweating out.
Because your cells have stuff in them. Ions, molecules, hormones, hemoglobin, all the things they use to do work and reproduce. Water travels down a concentration gradient until the concentration equalises. Put a red blood cell into water and the water will travel into the cell until it bursts and the concentration of hemoglobin equalises. We don’t want to do this inside your body, hence using isotonic solutions.
We do the same when we take your blood out of your body, I always wash blood cells, or dilute blood using 0.8% saline.
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