How does the kidney make urine: secretion or filtration?

Positions

1. Secretion by the tubule

   Bowman (1842) held that the glomerulus lets out water, while the cells of the tubule secrete the substances that give urine its character, such as urea and uric acid. Heidenhain (1874) revived the idea with dye experiments: in animals whose blood pressure had been lowered so that filtration should stop, injected dye still appeared in the tubular cells and lumen, which he read as active secretion.

   Sources

   1. Bowman 1842, On the structure and use of the Malpighian bodies of the kidney (Phil Trans, OCR) Primary
   2. historyofmedicine.com (Garrison-Morton derived) entry on Rudolf Heidenhain, Versuche über den Vorgang der Harnabsonderung, 1874 Secondary
   3. Peña Rodríguez 2018, Acta Med Grupo Angeles 16(S1):S7-S15 (history of renal physiology) Secondary

2. Filtration, then reabsorption

   Ludwig (1843) proposed that the pressure of the blood forces a protein-free fluid out through the glomerulus by physical filtration, and that the tubules concentrate it: urine formation explained by physics and chemistry alone. Cushny (1917) built his 'modern view' on filtration of plasma without its protein and reabsorption in the tubules, and denied tubular secretion.

   Sources

   1. historyofmedicine.com (Garrison-Morton derived) entry on Carl Ludwig, Beiträge zur Lehre vom Mechanismus der Harnsecretion, Marburg, N. G. Elwert, 1843 Secondary
   2. Peña Rodríguez 2018, Acta Med Grupo Angeles 16(S1):S7-S15 (history of renal physiology) Secondary
   3. Cushny, The Secretion of the Urine (London, 1917), archive.org scan Primary

3. Both: filtration, reabsorption and secretion

   The account settled in the 1930s keeps all three. Homer Smith described the kidney as both a filter and a secretory organ, so Bowman and Heidenhain were partly right and Cushny's denial of secretion was corrected.

   Sources

   1. Wikipedia: Homer W. Smith (listed in research/02 §11) Secondary

Evidence

For about eighty years both camps argued from indirect evidence: anatomy, dyes, and the effects of changing blood pressure. Wearn and Richards supplied the first direct sample. Fluid drawn from the capsule of a single frog glomerulus contained sugar and chloride and no detectable protein, while bladder urine taken at the same moment contained neither sugar nor chloride. That pointed to a protein-free filtrate at the head of the nephron and to reabsorption along the tubule. Their colleague Carl Schmidt judged that it suggested, but did not prove, filtration, and the objection that a frog is not a mammal, raised at the first report in December 1921, was met only by later work: inulin clearance in man in 1935 and mammalian micropuncture from 1941.

Current standing

Settled in favour of a combined account: glomerular filtration, tubular reabsorption and tubular secretion. Inulin, introduced by Richards and used by Homer Smith, gave a measure of the filtrate, and inulin clearance remains the reference standard for glomerular filtration rate. The film ends the quarrel with both sides half right.

All sources