The role of the collecting duct in urinary concentration.

In summary, the three major segments of the collecting duct subserve three different functions in the urinary concentrating mechanism. The main function of the cortical collecting tubule is to raise the fractional solute contribution and absolute concentration of urea in fluid that it delivers to the outer medullary collecting duct. The function of the outer medullary collecting duct is to raise further the absolute intraluminal urea concentration. Finally, the inner medullary collecting duct has two major functions in urinary concentration: first, it adds net urea to the papillary interstitium, and second, it allows the generation of maximally concentrated urine due to osmotic water equilibration. Indeed, the urine osmolality can rise to levels higher than the papillary interstitial osmolality as a consequence of inequalities of the reflection coefficients of urea and sodium chloride.