Mechanism of urine concentration: overview of the passive model. Several models of urine concentration have been put forth by investigators. The passive model of urine concentration described by Kokko and Rector [3] is based on permeability characteristics of different parts of the nephron to solute and water and on the fact that the active transport is limited to the thick ascending limb. 1) Through the Na+, K+, 2 Cl cotransporter, the thick ascending limb actively transports sodium chloride (NaCl), increasing the interstitial tonicity, resulting in tubular fluid dilution with no net movement of water and urea on account of their low permeability. 2) The hypotonic fluid under antidiuretic hormone action undergoes osmotic equilibration with the interstitium in the late distal tubule and cortical and outer medullary collecting duct, resulting in water removal. Urea concentration in the tubular fluid rises on account of low urea permeability. 3) At the inner medullary collecting duct, which is highly permeable to urea and water, especially in response to antidiuretic hormone, the urea enters the interstitium down its concentration gradient, preserving interstitial hypertonicity and generating high urea concentration in the interstitium.

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FIGURE 1-4 (continued)

4) The hypertonic interstitium causes abstraction of water from the descending thin limb of loop of Henle, which is relatively impermeable to NaCl and urea, making the tubular fluid hypertonic with high NaCl concentration as it arrives at the bend of the loop of

Henle. 5) In the thin ascending limb of the loop of Henle, NaCl moves passively down its concentration gradient into the intersti-tium, making tubular fluid less concentrated with little or no movement of water. H2O—water.

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