Ascending thin limb

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... 

The thick ascending limb of the loop of Henle actively reabsorbs NaCl from the lumen (about 35% of the filtered sodium), but unlike the proximal tubule and the thin limb, it is nearly impermeable to water. Salt reabsorption in the thick ascending limb therefore dilutes the tubular fluid, leading to its designation as a "diluting segment." Medullary portions of the thick ascending limb contribute to medullary hypertonicity and thereby also play an important role in concentration of urine. 

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). 

Fig. 2.9. Diagram of the structural organization of the kidney and its relationships to the zones of the kidney. CD, collecting duct CATT, connecting tubule DCT, distal convoluted tubule PT, proximal tubule TAL, thick ascending limb TL, thin limb of Henle s loop...

Thin ascending limb Fluid entering is hypertonic. The limb is impermeable to water but ion transport does occur, which causes the urine osmolarity to fall. 

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.)... 

At the boundary between the inner and outer stripes of the outer medulla, the proximal tubule empties into the thin descending limb of Henle s loop. Water is extracted from the descending limb of this loop by osmotic forces found in the hypertonic medullary interstitium. As in the proximal tubule, impermeant luminal solutes such as mannitol oppose this water extraction. The thin ascending limb is relatively water-impermeable. 

FIGURE 29—4 Mechanisms by which vasopressin increases the renal conservation of water. IMCD, inner medullary collecting duct TAL, thick ascending limb VRUT, vasopressin-regulated urea transporter. Thick and thin arrows denote major and minor pathways, respectively. 

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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