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Name: Abbie
Status: student
Grade: 6-8
Location: OK
Date: September 2008

Why can't humans drink salt water from the ocean?

A complete answer to this "simple" question gets pretty complicated because the answers are not so "simple": First, ocean "salt" water isn't just salt, there are other dissolved chemicals that can be toxic, that's the simple part. Second, many (maybe most) functions within the body depend upon the transfer of water across cell walls and membranes. Over the eons, the human body has evolved in such a way that the difference in concentration maintains a balance within certain limits. If that concentration difference becomes either too great, or not great enough, the result is for the body to reestablish that balance. Simple examples of this response is getting "thirsty" after eating salty foods, or after intense physical activity. In addition, it is not only just the amount of "salt" but the balance of various salts -- sodium, potassium, and calcium -- are examples. When out of balance thirst can result, muscle cramps develop, and many body functions no longer function properly. One (over)-simplified example is the absorption of water across the digestive tract cell walls. If the salt content of the intake water is too high, water flows, not from the digestive tract into the rest of the body, but just the opposite -- transfer of water from the body into the digestive tract. This process, on a limited controlled scale, is the mechanism by which some electrolytic laxatives work, such as magnesium sulfate. The body also requires a balance of various ions that trigger nerve cells. If not properly balanced the consequences can be lethal.

A complete explanation to your question could be, and probably has been, the topic of books. The above are just a few of the possible consequences.

Vince Calder

Table salt is only one of many "salts" which are specific types of chemicals that dissolve in water. The ocean has many types of salts dissolved in the ocean water. The living body has to maintain a certain amount of many salts, including table salt, and this is one of the kidney's jobs; to help regulate these desirable salt levels. The kidney does this by filtering and flushing the excess salts with water. Kidney also works to keep the water level in the body at its correct amounts. Ocean water has so much salt dissolved in its water that the kidney can not flush the excess salt with the amount of water the ocean water provides. The kidney demands more water to flush the excess salt and makes the person very thirsty so they will drink more water to help the kidney. Drinking more ocean water makes this problem worse and the kidney fails to keep the salts and water levels correct; this is serious and will lead to death.

Fresh water does not have excessive levels of salts and allows for the kidney to keep the levels of salts and the amount of water in the body at correct levels. This is why it is important to keep your body hydrated, especially during activities in which sweat and loss of water from the body is taking place.

Excellent Question.

Steve Sample

Water is the "solvent" that biochemical activity occurs in, and our cells require moderate salinity in order to function properly. The electrolytes (ionized salts) in the cells provides a means of moving body fluids and providing electrical nerve impulses, and the amount of salt needs to be "about right". Too much salinity can kill you. Oddly, too much *fresh* water can also kill you (do an internet search on "hyponatremia" or "water poisoning"), but it's very rare. So the fundamental problem is that sea water is around three times saltier than the insides of our cells, and a steady intake of ocean water would be lethal.

Our kidneys can excrete some salt, but humans have kidneys that are not very good at excreting salt in our urine (likewise our skin in excreting high salinity sweat).

In contrast, other animals have various solutions to salt regulation. Marine invertebrates (e.g., jellyfish) possess unusually salty cells, so they are at home in the ocean. "Tube-nose" sea-birds (e.g., Albatross) and Penguins excrete salt crystals out their noses, due to a lack of fresh water and "crummy" kidneys. The Kangaroo Rat can produce very salty urine with specialized kidneys. Most fish have body salinity similar to ours; in order to live in fresh or ocean water, they use a number of specialized adaptations to save or discard salt. So there you are. We're just not good at eliminating large amounts of salt.

As a side note, there has been speculation that we are slightly salty because we evolved from the ocean. The truth is, our very early ancestors (early fishes) are thought by most scientists to have evolved in fresh or brackish water, not the ocean. Therefore, you'd expect them (and us) to have non-salty cells, but their cells need some salt to function, so they are (and we are) slightly salty.

Paul Bridges

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