How can seawater dehydrate you?

Answer 1

Seawater contains sodium or salt. The amount of salt in sea water is several times higher than what your body needs. Your body absorbs the salt into your bloodstream which raises the salt concentration level or serum level of your blood. This triggers 3 things to happen. First, it causes the water in other cells in your tissue to rush to the blood stream to try to dilute the salt so now your tissue (muscles, skin, etc) is now dehydrated. The salt level also triggers your brain to tell you to feel thirsty so you will drink more fluid to help dilute the salt. If you are stranded at sea, you will become even more thirsty than you were before! Third, the brain tells the kidneys not to produce as much urine so you don't pee out the water and the fluid that is now rushing to your bloodstream to try to dilute the salt. But because the salt content is so high in seawater, it is almost impossible for the body to dilute that amount of salt so you just keep loosing fluid from your tissue and your kidney stop making pee altogether. Your brain is very sensitive to salt and even the smallest amount of difference of your salt level in your blood causes the brain to start doing very strange things so you also go crazy. Now, your tissue is dehydrated, your kidneys are not working so toxins are building up in your body and you are still very, very thirsty!