At first, it’s what daydreams are made up. Adrift on a lifeboat, you’re faced with drinking seawater or death. Or, your rational brain starts to wonder; if there’s so much ocean, and we’re made of salt, then why can’t we drink sea water?
The short answer is: we can’t. We would die.
But why would we die? The answer lies in how we’ve evolved into mammals.
While we all ultimately came from the oceanic depths, our physiology changed once we embarked on land. Our bodies had to contain water on their own, and continued to rely on salts such as sodium to maintain cellular health, transport nutrients, keep our hearts beating, and so on. Ultimately, we adapted a physiology quite different from our aquatic (non-mammal) cousins.
Most important for this article, our concentration of salts changed. Our cells (and blood) have a concentration of sodium of about 9 grams per 1,000 grams of water . This is enough to maintain our cellular health. If our diets bring in more sodium (and they usually do), then it’s up to our kidneys to filter out and send the excess salt out of our bodies, in our urine. Marine mammals do this, too; it’s been thought that dolphins, whales and seals drink seawater, but it’s more likely they don’t. Their kidneys, in fact, are specially adapted to excrete highly saline urine. And they have to; the ocean’s salinity is about 35 grams of salt per 1,000 grams of water. That’s more than three times the salinity of us!
So, what happens if we drink a lot of sea water?
Since sodium concentrations are key to maintaining the right water balance in cells, this much sodium will cause water to rush out of our cells. All of our cells. In addition, our over-salted blood carries this salt to the kidneys (made up of already-stressed cells), which normally could filter it out and carry it off. But faced with these salt concentrations, the kidneys can’t work hard enough to reduce 35 grams of salt to 9 grams. They start shutting down. Once kidney failure is prolonged enough, we die.
These large concentrations of salt, meanwhile, would have affecting other parts of the body, too. Blood pressure will rise, and heart rates will rise, causing even more water to disappear from our cells and bodies. Dizziness, loss of consciousness, and impaired judgement (even more so than the decision to drink seawater) happen next.
Not a great way to go.
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