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What if you drink saltwater?

Ever daydream from your windowless office about being stranded on a deserted island? Well, when your genie comes around, you might want to pop in a clause regarding running water.

If your paradise doesn't have a convenient source of freshwater, those lapping ocean waves won't be lowering your blood pressure for long. In fact, as you scavenge "Survivor" style for liquid from fruit sources, you may find that the expansive aquamarine sea is mocking you. Because you can't drink any of it. Seventy one percent of our planet's surface is wet with water, yet most of it is ocean water that we can't consume, thanks to salt [source: McLamb].

Saltwater doesn't sound as deadly as, say, taking your chances on puffer fish sushi. After all, our bodies depend on both water and salt to function. Water, the universal solvent for proteins and the like, is essential for chemical reactions that help us metabolize food, use our muscles, pump our blood and even think. Our cells also depend on salt, which exists as sodium and chloride ions in our extracellular fluids, for those same chemical reactions. So, what's the problem? If anything, drinking saltwater sounds efficient, not deadly.

Although consuming a little salt is essential for our physiological well-being (and for the fries to taste good), too little or too much wreaks havoc in our bodies [source: Stoppler]. For humans, that magic number is 9. That's the salinity, or the weight in grams of salt dissolved in 1,000 grams of water, of human blood [source: Ocean Health]. This means that every 1,000 grams of fluid contains 9 grams of salt and 991 grams of water. In medicine, fluids that have the same salinity as blood are referred to as isotonic [source: Ocean Health]. When we consume too much salt, we excrete the excess in our urine to keep our bodily fluids isotonic.

Saltwater is a hypertonic fluid, or one that contains more salt than human blood, and it has a salinity of 35 [source: U.S. Geological Survey]. As you're about to see, drinking extremely hypertonic fluids such as seawater throws the body's coping mechanism into disarray.

The Salinity Sea-Saw

How is it that sea-loving marine life doesn't shrivel up and die from all the salinity? Those clever creatures deal with ocean salinity in two ways.

  1. They act as osmotic conformers. Marine plants and invertebrates have no mechanism to control osmosis, so their cells are the same salinity as their environment (35 for ocean dwellers). That means saltwater intake doesn't disrupt their physiological equilibrium.
  2. They're osmotic regulators. Most fish, reptiles, birds and mammals control osmosis in a variety of ways. For instance, salmon use specialized cells on their gills (called chloride cells) to cope with osmosis. Chloride cells can excrete excess salt, allowing the fish to take in water without dehydrating.
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Potential Effects of Drinking Saltwater

Have you ever been minding your own business on an elevator when an aggressively perfumed person stepped on? What happened? Did the Lady Stetson/Drakkar Noir stay on the person? Nope, it wafted all over the elevator so that everyone could smell it. That's diffusion in action. This net transport of matter from a region of high concentration to a region of lower concentration is happening all the time [source: Gross].

When it comes to diffusion and saltwater though, human cells have biological membranes, which can prevent salt from freely waltzing into our cells. Although our bodies can normalize sodium and chloride concentrations to an extent, dealing with extremely high concentrations of salt in the blood is challenging. That's because a cell's membrane is semipermeable -- although sodium, chloride and other substances may not be able to easily diffuse in and out of the cell, water can. When the salt concentration is higher on the outside of our cells than on the inside, water moves from the inside to the outside of the cells to correct the imbalance. The attempt to equalize the concentrations of matter on both sides of a semipermeable membrane is called osmosis.

If you're consuming seawater, the results of osmosis are spectacularly disastrous. Remember the salinity of seawater is almost four times that of our bodily fluids. If gone unchecked, the net transfer of water from the inside of your cells to the outside will cause the cells to shrink considerably -- and shrinkage is never good.

Unless you drink a lot of freshwater, the body's regulatory mechanism in this situation is potentially fatal. With seawater, the change in sodium concentration outside our cells is the main culprit. In order to regain an isotonic state, a must for cell survival, the body attempts to eliminate the excess sodium from its extracellular fluids. It secretes urine. However, human kidneys can only produce urine that's slightly less salty than saltwater. So, in order to remove the extreme amount of sodium taken in by saltwater, we urinate more water than we actually drank. And dehydration sets in.

So, if you're guzzling seawater, you actually aren't taking in any water but are incurring a net loss, leading to depleted body fluids, muscle cramps, dry mouth, and yes, thirst.

The body tries to compensate for fluid loss by increasing the heart rate and constricting blood vessels to maintain blood pressure and flow to vital organs. You're also most likely to feel nausea, weakness and even delirium. As you become more dehydrated, the coping mechanism fails. If you still don't drink any water to reverse the effects of excess sodium, the brain and other organs receive less blood, leading to coma, organ failure and eventually death.

Of course, consuming small amounts of saltwater won't kill you. The take home message is clear, though: Salt and water are best consumed separately -- and any salt intake should be accompanied by plenty of freshwater.

Lots More Information

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Sources

  • Gross, Cliff, Josh DeZeeuw and Ted Simpao. "Awesome Osmosis." Marine Discovery. The University of Arizona. April 27, 2001. (Jan. 30, 2012) http://marinediscovery.arizona.edu/lessonsS01/blennies/2.html
  • Marine Science. "Seawater Composition." Oct. 8, 2008. (Jan. 30, 2012). http://www.marinebio.net/marinescience/02ocean/swcomposition.htm
  • McLamb, Eric. "Earth at a Glance." Ecology. Sept. 14, 2011. ( Feb. 4, 2012) http://www.ecology.com/2011/09/14/earth-glance/
  • Ocean Plasma. "Chemistry of Seawater." Ocean Health. (Jan. 30, 2012) http://oceanplasma.org/documents/chemistry.html
  • Stoppler, Melissa Conrad. "Electrolytes." MedicineNet. ( Feb. 4, 2012) http://www.medicinenet.com/electrolytes/article.htm
  • U.S. Geological Survey. "Thirsty? How 'bout a cool, refreshing cup of seawater?" Water Science for Schools. Dec. 22, 2011. (Jan. 30, 2012) http://ga.water.usgs.gov/edu/drinkseawater.html
  • Wedro, Benjamin. "Deyhdration." MedicineNet. ( Feb. 4, 2012) http://www.medicinenet.com/dehydration/article.htm