FIG. 41.20

How does the mammalian kidney produce concentrated urine?

BACKGROUND The mammalian kidney produces urine that is more concentrated than the blood. How it concentrates urine was one of the most perplexing problems in renal physiology.

HYPOTHESIS One hypothesis is that the kidney actively transports water out of the collecting ducts as the filtrate leaves the kidney. Another, first suggested by the Swiss chemist Werner Kuhn in the early 1950s, is that the loop of Henle creates a concentration gradient from the cortex to the medulla by a countercurrent multiplier mechanism. According to this model, the solute concentration is low in the cortex and high in the medulla, so water leaves the collecting ducts passively as it moves from the cortex to the medulla.

PREDICTION A prediction of Kuhn’s hypothesis is that the solute concentrations of fluid taken at the same level from the medulla, loop of Henle, and collecting ducts should be similar to one another, and higher than that of the blood. If, instead, water is simply pumped out of the collecting ducts, then the fluid in the collecting ducts should be more concentrated than that in the loop of Henle.

EXPERIMENT American physiologists Carl W. Gottschalk and Margaret Mylle tested Kuhn’s hypothesis. They used innovative micropuncture techniques in hamsters to measure the solute concentration of fluid taken from the bend of the loop of Henle and the collecting ducts at the same level.

RESULTS Their results, shown in the table, are consistent with the predictions of Kuhn’s hypothesis. That is, the solute concentrations of the fluid in the loop of Henle and in the collecting ducts are similar to each other and higher than that of the blood.

CONCLUSION These results are consistent with a model in which concentrated urine is produced by first generating a concentration gradient between the cortex and the medulla, and then allowing water to move out of the collecting ducts by osmosis.

FOLLOW-UP WORK Similar measurements in other areas of the kidney and in other mammals and birds supported the model.

SOURCES Gottschalk, C. W., and M. Mylle. 1958. “Evidence That the Mammalian Nephron Functions as a Countercurrent Multiplier System.” Science 128:594; Gottschalk, C. W., and M. Mylle. 1959. “Micropuncture Study of the Mammalian Urinary Concentrating Mechanism: Evidence for the Countercurrent Hypothesis.” American Journal of Physiology 196 (4):927–936.