Micropuncture techniques have been used to investigate the role of each nephron segment in the river lamprey. The proximal segment reabsorbed no more than 10% of filtered water and tubular fluid here was iso-osmotic to plasma, at about 250 m-osmol. Further water reabsorption occurred in later nephron segments so that 44% of all filtered water was reabsorbed. Dilution of tubular fluid began in the ascending limb of the nephron loop and continued in the distal and collecting segments, so that 96% and 97.5% of filtered sodium and chloride, respectively, was reabsorbed by the kidney. Lampreys produce 337 ml kg−1 day−1 of a dilute urine (31.2 m-osmol) and the urinary ducts appear not to modify the composition of this urine.

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