Descending thin limb

Four factors contribute to the ability to concentrate urine (1) Active reabsorption of Na+, K+, and CF without water reabsorption by the thick ascending limb of the loop of Henle results in interstitial hypertonicity and hypoosmotic tubular fluid. (2) Selective permeability to water, but not small electrolytes, in the descending thin limb of the loop of Henle allows passive reabsorption of water, facilitated by interstitial hypertonicity. (3) Relatively low medullary blood flow maintains medullary hypertonicity, allowing continued elaboration of concentrated urine. (4) In the presence of ADH, the distal tubule and collecting ducts are permeable to water so that water may diffuse out of the tubular lumen into the medullary, and papillary inter-stitium. Because of the ability of the thick ascending limb of the loop of Henle to move solutes but not water into the medullary interstitium, the medullary, and papillary interstitium are hyperosmotic and hypertonic compared to plasma and cortical interstitium (Figure 29.5). 

CNS central nervous system DTL descending thin limb... 

At least 11 different mammalian AQP have now been identified, of which seven (AQPl, -2, -3, -4, -6, -7, -8) are expressed in the Iddney. Many of these also have extra-renal expression sites (e.g, AQPl may be unportant in fluid removal across the peritoneal membrane). Two asparagine-prohne-alanine sequences in the molecule are thought to interact in the membrane to form a pathway for water translocation. AQPl is found in the proximal tubule and descending thin limb of the loop of Henle and constitutes almost 3% of total membrane protein in the kidney. It appears to be constitutively expressed and is present in both the apical and basolateral plasma membranes, representing the entry and exit ports for. water transport across the cell, respectively. Approximately 70% of water reabsorption occurs at this site, predominantly via a transcellular (i.e., AQPl) rather than a paracellular route. Water reabsorption in the proximal tubule passively follows sodium reabsorption, so that the fluid entering the loop of Henle is still almost isosmotic with plasma. 

The terminal portion of the proximal tubule leads into the descending thin limb of Henle in the region corresponding to the demarcation of outer and inner stripes of the outer zone of the medulla. The thin limb loops upward to form the ascending thin limb of Henle, which differentiates into the ascending thick limb of Henle at the intersection of the inner zone of medulla and the inner stripe of the outer medullary zone. This is characteristic of juxtamedullary nephrons that have long loops of Henle. In contrast, as noted earlier, cortical nephrons have short loops of... 

Pars descendans Intermediate Descending thin limb (DTL) DTL cells ... 

At the boundary between the inner and outer stripes of the outer medulla, the thin limb of Henle s loop begins. Water is extracted from the thin descending limb of the loop of Henle by osmotic forces created in the hypertonic medullary interstitium. As in the proximal tubule, impermeant luminal solutes such as mannitol oppose water extraction. 

PCT is proximal convoluted tubule, DL is descending limb of the loop of Henle, Thin AL is thin ascending limb of the loop of Henle, Thick AL is thick ascending limb of the loop of Henle, DCT is distal convoluted tubule and CD is collecting duct. (This figure is reproduced with permission from Fundamental Principles and Practice of Anaesthesia, P. Hutton, G. Cooper, F. James and J. Butterworth. Martin-Dunitz 2002 pp. 487, illustration no. 25.16.)... 

The thick ascending limb (TAL) of the loop of Henle actively reabsorbs NaCI from the lumen (about 25% of the filtered sodium), but unlike the proximal tubule and the thin descending limb of Henle s loop, it is nearly impermeable to water. Salt reabsorption in the TAL therefore dilutes the tubular fluid, and it is called a diluting segment. Medullary portions of the TAL contribute to medullary hypertonicity and thereby also play an important role in concentration of urine by the collecting duct. 

It is likely that transport also occurs in the highly lithium permeable thin descending limb along the osmotic gradient to the hyperosmolar inner medulla... 

Mannitol (IV only) inhibits water reabsorption in the proximal convoluted tubule (PCT) (main site), the thin descending limb of the loop of Henle, and the collecting ducts. It increases urine volume, preventing anuria in hemolysis and rhabdomyolysis, and facilitates elimination of toxic drugs (e.g cisplatin). Similar osmotic actions in the ECF of other tissues —>4- intraocular and intracerebral pressure. 

Aquaporins are a family of water channel proteins that allow water molecules to cross biological membranes. Of the 10 cloned mammalian aquaporins, at least 7 are found in the kidney. Aquaporin I is present in the apical and basolateral membrane of the proximal tubule and in the thin descending limb. Aquaporin 2 resides in the apical membrane and WCVs of the collecting-duct principal cells, whereas aquaporins 3 and 4 are present in the basolateral membrane of principal cells. Aquaporin 7 is in the apical brush border of the straight proximal tubule. Aquaporins 6 and 8 are... 

The U-shaped portion of a renal tubule lying between the proximal and distal convoluted portions. It consists of a thin-descending limb and a thicker ascending limb. 

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