Short and long loop nephrons.

The explanation for the necessity to have both short and long loop nephrons for urinary concentration is unknown but may represent nature's resolution of conflicting ideal conditions for maximum urinary concentration. Ideally, one would like the thick ascending limb to extend throughout the entire medulla to the papillary tip and be supplied by a blood flow vigorous enough to provide oxygen and remove waste products as rapidly as needed. One would also like to have a progressively smaller volume of tissue to be concentrated toward the papillary tip to lessen the osmotic work required and a highly efficient vascular exchange system to sequester the medullary interstitial solute effectively. But the same efficiency of countercurrent exchange of oxygen causes the inner medulla to have a relatively low oxygen content. The presence of the thin loops of Henle in the inner medulla may represent a compromise between these conflicting ideals. The papilla tapers to a low mass, which allows a mechanism requiring only a modest energy supply to increase the tonicity of the interstitium enormously. The reduced work requirement obivates the need for thick ascending limbs to extend into the papilla where they would be highly vulnerable to anoxia. The outer medulla with its larger mass and thick ascending limbs supplied by a high blood flow can initiate the operation to reduce the volume of fluid and solute to be concentrated, and at the same time carry out other functions required of the filtration-reabsorption kidney.(ABSTRACT TRUNCATED AT 250 WORDS)