Micropuncture study of the mammalian urinary concentrating mechanism: evidence for the countercurrent hypothesis.

927-936. = 959. -The osmolality was determined of fluid collected by micropuncture from proximal and distal convolutions, loops of Henle, collecting ducts and vasa recta of kidneys of various rodents with and without osmotic diuresis. Proximal tubular fluid was isosmotic; in the presence of antidiuretic hormone, early distal fluid was hypo-osmotic due to the prior reabsorption of sodium chloride, and late distal fluid again isosmotic. The hyperosmotic concentration of the urine is established in the collecting ducts, apparently as a consequence, in part at least, of the hyperosmotic reabsorption of sodium chloride in the loops of Henle. Fluid from the bends of loops of Henle, and from collecting ducts and vasa recta at the same level were equally hyperosmotic, consistent with the hypothesis that the mammalian nephron functions as a countercurrent multiplier system. The vasa recta are believed to play an important role in the concentration of the urine by functioning as countercurrent diffusion exchangers.