"Water, water, every where, nor any drop to drink" is a phrase from the Rime of the Ancient Mariner by Samuel Taylor Coleridge. This poem describes an ocean ship unable to sail to land and running out of its supply of fresh water; sailors have long known that drinking seawater cannot prevent death from dehydration. What would result if the sailors attempted to alleviate their dehydration by

Question

drinking seawater? Justify your answer by describing kidney physiology. Why wouldn't reflexes, in response to dehydration, fully compensate? What does this illustrate about the force allowing kidneys to retain water under more normal conditions?

What will be an ideal response?

Answer 1

The osmolarity of seawater is higher than that of the kidney medulla, thus the osmotic gradient that normally
allows net reabsorption of water does not exist. Seawater osmolarity is largely a result of permeant ions such as
sodium and chloride, so ingested seawater raises plasma osmolarity when it is absorbed. When the kidneys form a
filtrate of this high-osmolarity solution, the descending limb fails to reabsorb water because the kidney medulla
has a lower osmolarity than the filtrate (opposite of normal), along its entire length, and in fact the medulla would
lose water as it moved into the descending limb. While ions would be reabsorbed by the ascending limb as usual, it
is not enough to drive sufficient water reabsorption. Reflexes such as secretion of water-preserving hormones are
ineffective because, ultimately, water reabsorption depends only on osmosis, and the normal osmotic gradient has
been disrupted.