Obligatory Urine Volume
The maximal concentrating ability of the kidney dic-tates how much urine volume must be excreted each day to rid the body of waste products of metabolism and ions that are ingested. A normal 70-kilogram human must excrete about 600 milliosmoles of solute each day. If maximal urine concentrating ability is 1200 mOsm/L, the minimal volume of urine that must be excreted, called the obligatory urine volume, can be calculated as
This minimal loss of volume in the urine contributes to dehydration, along with water loss from the skin, respi-ratory tract, and gastrointestinal tract, when water is not available to drink.
The limited ability of the human kidney to con-centrate 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 per cent, with an osmolarity between about 1000 and 1200 mOsm/L. Drinking 1 liter of seawater with a con-centration 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 con-centrated. 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. However, a shipwreck victim’s pet Australian hopping mouse could drink with impunity all the seawater it wanted.
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