Renal medulla

The renal medulla is the inner region of the kidney, lying beneath the renal cortex and organized into renal pyramids. It contains portions of the Loop of Henle, collecting ducts and the renal vasa recta. The arrangement supports the production of urine more concentrated than blood plasma.^[ref]

Along the renal medulla, the surrounding fluid generally becomes more concentrated toward the renal papilla. Countercurrent multiplication involving the Loop of Henle helps establish this gradient; exchange in the renal vasa recta helps preserve it. A water-permeable collecting duct can then lose water by osmosis while passing through the concentrated surroundings.

In 1959, Carl W. Gottschalk and Margaret Mylle measured osmolality in tiny fluid samples collected by renal micropuncture. Oil-filled glass micropipettes sampled particular locations; freezing-point depression supplied a concentration measurement. Fluid from loop bends, collecting ducts and renal vasa recta at the same medullary level was similarly hyperosmotic. This supported a shared local concentrating environment and a countercurrent interpretation, rather than concentration occurring only inside one isolated tube.^[ref]

The experiment connects location to function: a diagram showing a Loop of Henle merely bending back is incomplete without the measured concentration differences along the surrounding tissue.