Osmotic diuresis before Homer W. Smith: a winding path to renal physiology

It was towards the end of the 18th century that a curiosity arose about what controlled the flow of urine. It was noted that though a patient's kidneys might be destroyed, the flow of urine continued until death. This puzzling phenomenon might have been understood had Ségalas's and Wöhler's observations been properly appreciated, that an extra load of urea, or any other substance that is excreted by the kidney, causes a diuresis. Nevertheless, these experiments were forgotten, and because of Bowman's and Ludwig's studies fluctuations in urine flow were ascribed to changes in blood pressure. It was not until 1870 to 1880 that Ustimowitsch, Falck and Richet stressed the role of urinary solutes on urine flow. Around 1900, Cushny also studied the diuretic effect of different solutes excreted in the urine and actually described what is now called an osmotic diuresis, though he did not use the term. These studies enabled him to deliver his "modern view" of the Secretion of the Urine. This was the first coherent concept which attempted to explain the formation of the urine and the regulation of its composition according to a pressure of a selective tubular reabsorption controlled by the needs of the "Milieu Intérieur." When glomerular and tubular functions became quantifiable, Homer W. Smith's disciples were able to define the exact nature of an osmotic diuresis and that explained the paradox of polyuria and renal failure.