So one of our readers asked us this question the other day: Why does putting salt water (saline) into our blood stream rehydrate us, but drinking salt water can harm us?
It has to do with how salty the water is (i.e. the osmolarity of the fluid). The saline usually used in IV drips for rehydration is close to isotonic – that is, the amount of ‘salt’ is approximately the same as what’s in the blood stream. This prevents “diluting” the blood (i.e. causing hyponatremia etc) and/or causing excessive fluid redistribution to the various compartments.
In a way, it’s similar to how sports drinks can have some salt in them without causing dehydration.
On the other hand, the salt water you’re probably thinking of is something like sea water, which is hypertonic. Although drinking it doesn’t really ‘harm us’ per se, it doesn’t rehydrate us. This is because urine has to be excreted in order to maintain osmolarity.
Certain receptors detect when there’s too much ‘salt’ in the blood and concentrated urine is excreted (bringing the osmolarity back down to normal). However, there’s a limit to how concentrated our kidneys can make urine, so the amount of water we gained from drinking sea water is less than the amount of urine that had to be excreted – thus increasing the level of dehydration.
In technical terms, saline is usually 0.9% sodium chloride in water whereas ‘salt water’ from the ocean is on average, approximately 3.5% NaCl in water.