Re: How do cells in the renal medulla tolerate such high osmolarity?

  

    
As fluid flows up the ascending limb of the loop of
        Henle salts, but not water, are pumped out.  
      
These salts, are taken up by the descending limb, leading
        to very high osmolarities.  
      
a great web site on kidney function  
      
To resist the effects of this hyperosmotic environment,
        the cells within the descending loop and turn accumulate various organic
        osmolytes.  
      
 
    
    
 
    
 
  


  

    
 
    
 
    
These osmolytes include inositol,
        betaine (trimethylglycine), and sorbitol.  
      
The cells absorb inositol and betaine from their surroundings.
         
      
In response to osmotic stress, they increase their expression
        of the  gene
        encoding the protein aldose reductase, which converts
        glucose to sorbitol.
         
      
The intracellular accumulation of these  compounds
        counteracts the high extracellular salt concentration, protecting the
        cells.