The kidneys are the filters that separate waste materials from the blood, which are stored in the form of urine in the bladder, ready for expulsion to the outside.
Although the kidneys can do their job without us drinking a lot of water, they are limited in that they are unable to produce urine with a salt concentration greater than 2% . Sea water contains 3% salt, so if we drink a liter, our kidneys will need at least a liter and a half of pure water to dilute all the salt. To achieve this, they will be forced to withdraw an extra half liter of water from our body, with the consequent increase in dehydration and the sensation of thirst. This explains why it is preferable not to drink at all to drinking sea water and also the fact that many drinks consisting mainly of water do not quench thirst.
