Why severe dehydration occurs if one attempts to drink seawater?

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We were asked this question in physiology class today. No one could answer the question and later the teacher explained the answer, but I didn't quite get it. Can anyone please explain to me in short?

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The limited ability of the human kidney to concentrate the urine to a maximal concentration of 1200 mOsm/L explains why severe dehydration occurs if one attempts to drink seawater. Sodium chloride concentration in the oceans averages about 3.0 to 3.5 %, with an osmolarity between about 1000 and 1200 mOsm/L. Drinking 1 liter of seawater with a concentration of 1200 mOsm/L would provide a total sodium chloride intake of 1200 milliosmoles. If maximal urine concentrating ability is 1200 mOsm/L, the amount of urine volume needed to excrete 1200 milliosmoles would be 1200 milliosmoles divided by 1200 mOsm/L,
or 1.0 liter.

Why then does drinking seawater cause dehydration?
The answer is that the kidney must also excrete other solutes, especially urea, which contribute about 600 mOsm/L when the urine is maximally concentrated. Therefore, the maximum concentration of sodium chloride that can be excreted by the kidneys is about 600 mOsm/L. Thus, for every liter of seawater drunk, 2 liters of urine volume would be required to rid the body of 1200 milliosmoles of sodium chloride ingested in addition to other solutes such as urea. This would result in a net fluid loss of 1 liter for every liter of seawater drunk, explaining the rapid dehydration that occurs in shipwreck victims who drink seawater.

Source: Textbook of Medical Physiology - Guyton and Hall

answered Jun 6, 2012 by admin Doctor of Medicine (9,784 points)
selected Jun 11, 2012 by Sulabh Shrestha
commented Apr 22, 2014 by wise azz
Ha, technically, you could drink enough salt water to finally extract enough water to live.  I say technically because I doubt anyone could actually do it, but if you could force yourself to drink 8 liters of salt water a day, at 1200mOsm/L you could extract 1 liter in usable water/day.  The above answer is actually off by a bit, the kidneys can pull enough water out of the tubules (especially the collecting tubule) to bring the osmolarity to around 1400mOsm/L.  This is our technicality.    

The body is obligated to get rid of 600mOsm/day, so if you add this to the 1200mOsm/l you drink with one liter of salt water, (i.e 1800mOsm/day) you actually lose .28L of water for the first liter of salt water you drink.  But since the initial loss included the obligated 600mOsm/day, every litter you drink on top of this would add 14.3% of usable water to the equation ( in other words, for every 1 liter of salt water we drink, we need to excrete .857 liters to balance out the salt we just drank)  Add enough, and you finally overcome that .28l that  we loose, and start adding positive values to the equation.  So 8 liters*1200mOsm/liter= 9600mOsm+600mOsm(obligate)= 10200mOsm/day needing excreting, which at a maximum urine osmolarity of 1400mOsm/liter= 7.08 liters of water lost to rid the body of the excess solute.  So, we end up peeing out 88.5% of the water we drink, but, we would technically be able to live ;)