Carl W. Gottschalk was a researcher in renal physiology who used renal micropuncture to investigate how the kidney handles fluid and concentrates urine. Working with Margaret Mylle, he developed experiments that connected conditions in individual renal tubules to the function of the whole kidney.[ref]
Their 1959 measurements found that fluid in loop bends, collecting ducts and renal vasa recta at the same medullary level was similarly hyperosmotic. This supported countercurrent multiplication and a shared concentrating environment in the renal medulla.[ref]
Micropuncture study of the mammalian urinary concentrating mechanism: evidence for the countercurrent hypothesis by Carl W. Gottschalk and Margaret Mylle. Abstract: comparison of fluid osmolality at matched medullary levels
Sampling the correct structure was itself part of the evidence. The memoir describes checking puncture locations with dye and microdissection, helping distinguish a renal tubule from a nearby blood vessel. A concentration measurement becomes biologically interpretable only when the source of the sample is secure.[ref]
Carl W. Gottschalk by Maurice B. Burg. Micropuncture studies: site verification and distinguishing loop fluid from blood
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