Determinants of the renal concentrating mechanism. Human kidneys have two populations of nephrons, superficial and juxtamedullary. This anatomic arrangement has important bearing on the formation of urine by the countercurrent mechanism. The unique anatomy of the nephron [1] lays the groundwork for a complex yet logical physiologic arrangement that facilitates the urine concentration and dilution mechanism, leading to the formation of either concentrated or dilute urine, as appropriate to the person's needs and dictated by the plasma osmolality. After two thirds of the filtered load (180 L/d) is isotoni-cally reabsorbed in the proximal convoluted tubule, water is handled by three interrelated processes: 1) the delivery of fluid to the diluting segments; 2) the separation of solute and water (H2O) in the diluting segment; and 3) variable reabsorption of water in the collecting duct. These processes participate in the renal concentrating mechanism [2].

1. Delivery of sodium chloride (NaCl) to the diluting segments of the nephron (thick ascending limb of the loop of Henle and the distal convoluted tubule) is determined by glomerular filtration rate (GFR) and proximal tubule function.

2. Generation of medullary interstitial hypertonicity, is determined by normal functioning of the thick ascending limb of the loop of Henle, urea delivery from the medullary collecting duct, and medullary blood flow.

3. Collecting duct permeability is determined by the presence of antidiuretic hormone (ADH) and normal anatomy of the collecting system, leading to the formation of a concentrated urine.

Determinants of the urinary dilution mechanism include 1) delivery of water to the thick ascending limb of the loop of Henle, distal convoluted tubule, and collecting system of the nephron; 2) generation of maximally hypotonic fluid in the diluting segments (ie, normal thick ascending limb of the loop of Henle and cortical diluting segment); 3) maintenance of water impermeability of the collecting system as determined by the absence of antidiuretic hormone (ADH) or its action and other antidiuretic substances. GFR—glomerular filtration rate; NaCl—sodium chloride; H2O—water.

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