The New York Review of Science Fiction Issue #296 April 2013 by Kevin J. Maroney

Author:Kevin J. Maroney
Language: eng
Format: epub
Publisher: Burrowing Wombat Press
Published: 2013-04-27T16:00:00+00:00

Of Blood and Salt and Sea Water

The sodium concentration of human blood is ~140 mmol/l (millimoles per liter); seawater is ~480 mmol/l; fresh water is only ~0.2–1.0 mmol/l. Sodium and the other abundant salts give human blood an overall salt concentration (osmolality) of ~280 mOsm/l (milliosmoles per liter) as compared to seawater at ~1,200 mOsm/l. The maximal urine concentrating ability is ~1,200 mOsm/l, but at this high concentration, 600 mOsm/l is needed for the obligatory excretion of urea and other salts. Therefore, if someone consumes 1 liter of seawater containing 1,200 mOsm of sodium, 1 liter of maximally concentrated urine could get rid of only 600 mOsm of sodium from seawater, requiring a second liter of urine in order to get rid of the other 600 mOsm of sodium. Some sodium will also be lost in sweat at a concentration that is similar to freshwater so it’s not a significant route for getting rid of excess sodium after consuming seawater. If the person is rapidly sweating out water in the sun, fresh water is needed to replace those losses.

Simply put, consuming 1 liter of seawater will force about 2 liters of urine output in order to maintain a near-normal serum sodium, thereby accelerating the dehydration. As the person becomes more dehydrated, the kidneys will no longer be able to excrete the excess sodium, and the blood sodium will rise more rapidly to critical levels. There is no question that exclusively consuming seawater must be avoided. On the other hand, mixing seawater with fresh water in a 40:60 ratio creates a sodium concentration of ~190 mmol/l as compared to the normal blood sodium of ~140 mmol/l. Such a mix of water will taste quite salty but in a well-hydrated person normal kidneys should be able to handle that sodium load without undue fluid loss. And so mixing seawater with fresh water may be a reasonable way of stretching a limited fresh water supply. But a seawater:fresh water ratio that gives a sodium concentration closer to 50 mmol/l would be even better as it mimics the concentration of athletic rehydration formulas. back to main article

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