= Renal medulla
{wiki}
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>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section25.3, Internal Anatomy and renal circulation]{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.<Micropuncture study of the mammalian urinary concentrating mechanism: evidence for the countercurrent hypothesis by Carl W. Gottschalk and Margaret Mylle>[Abstract; Methods p928: sampling apparatus and osmolality measurement]{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>.
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