Why is the sea water not drinkable?

Aha : Why doesn't seawater quench thirst?

Sea birds drink sea water. It's three to four times as salty as your blood, but with the help of special glands in your skull, you can remove the excess salt. If we were similarly equipped, all of mankind's water problems would be solved. But the sea water remains inedible for us.

The salt concentration in the sea can reach up to 39 grams per liter, while that of our blood is around nine grams per liter. Our body regulates the salt balance with the help of the kidneys. They consist of innumerable capsules and small blood vessels and are supplied with more blood than any other organ. On the way through this microfiltration system, the blood is cleaned and valuable components are recovered. The urine usually contains neither fats, proteins nor sugar. Most of the water is also retained.

However, the kidneys cannot produce any highly concentrated urine. The more salt we ingest, the more water is needed to flush it out. That is why we are more thirsty after a salty meal.

Shipwrecked people who drink a liter of sea water need about one and three quarters of a liter of fresh water to eliminate the excess salt. "To get rid of the salt, your body falls back on the water reserves in the cells," says Ulrich van Laak from the Naval Medical Institute of the Navy in Kronshagen near Kiel. The loss of water suffered in this way damages the cells, most rapidly those of the central nervous system. "Those affected become restless, begin to hallucinate and fall into a coma," says the doctor.

Drifting about in the open sea, there is a great temptation to give in to a feeling of thirst. But it is better to wait for rain, collect dew or flying fish. This is no joke. Occasionally a fish lands on the deck of a boat on the high seas. If you squeeze it, a lot of body fluid comes out - amazingly fresh water. Although fish live in the sea, their blood only contains nine grams of salt per liter. They owe this to their gills, which function like small seawater desalination plants. Thomas de Padova

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