Why do hospitals inject saline into your bloodstream instead of just water?
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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.
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