Figure 1917

Schematic diagram of loop-of-Henle thin-limb epithelial cells. Roman numerals (l-IV) identify the various segments of the epithelium and the region where they are found in the thin limb of the short and long loops of Henle. The diagrams of the epithelium do not include nuclear regions of the epithelial cells. (Modified from Madsen KM, Tisher CC. Kidney Hormones 1986;3:45-100.)

by reabsorbing more salts than water. The two limbs of the loop of Henle have different permeabilities and thus different functions:

• The thin descending limb of the loop of Henle is permeable, permitting free passage or equilibration of salt and water between the lumen of the nephron and the peritubular connective tissue. Because the interstitial fluid in the medulla is hyperosmotic, water diffuses out of, and salt diffuses into, the nephron at this site. The cells of this limb do not actively transport significant amount of ions; thus changes in osmolarity are the result of passive movement of water into the peritubular connective tissue and of salt and urea into the thin descending limb.

The thin ascending limb of the loop of Henle also has active function but allows passive diffusion of NaCl into the interstitium. CI" ion diffuses into the intersti-tium following its concentration gradient through the Cl~-conducting channels. Although the energy from ATP is required to open these channels, the movement of CI" is not an example of active transport and does not require Cl"-stimulated ATPase activity. Counter ions, in this case Na'1 (the majority) and K+, follow passively in order to maintain electrochemical neutrality. The hyperosmolarity of the interstitium is directly related to the transport activity of the cells in this limb of the loop of Henle. As in most pump systems, a counter ion, in this case Na+, follows passively to maintain electrochemical neutrality. Further, the thin ascending limb is largely impermeable to water, so that at this site, as the salt concentration increases in the interstitium, the interstitium becomes hyperosmotic and the fluid in the lumen of the nephron becomes hypoosmotic.

Distal Straight Tubule

The distal straight tubule is a part of the ascending limb of the loop of Henle

The distal straight tubule (thick ascending limb), as previously noted, is a part of the ascending limb of the loop of Henle and includes both medullary and cortical portions, with the latter located in the medullary rays. The distal straight tubule, like the ascending thin limb, transports ions from the tubular lumen to the interstitium. The apical cell membrane in this segment has electroneutral transporters (synporters) that allow CI", Na1, and K+ to enter the cell from the lumen. Na+ is actively transported across the extensive basal-lateral plications by the Na+/K+-ATPase pumps; CI" and I<' diffuse out from the intracellular space by the Cl" and K+ channels. Some K+ ions leak back into the tubular fluid throughout I<+ channels, causing the tubular lumen to be positively charged in respect to the interstitium. This positive gradient provides the driving force for the re-absorption of many other ions such as Ca2+ and Mg2+. Note that this significant movement of ions occurs without the movement of water through the wall of the distal straight tubule, resulting in separation of water from its solutes.

In routine histologic preparations, the large cuboidal cells of the distal straight tubule stain lightly with eosin, and the lateral margins of the cells are indistinct. The nucleus is located in the apical portion of the cell and sometimes, especially in the straight segment, causes the cell to bulge into the lumen. As noted above, these cells have extensive basal-lateral plications, and there are numerous mitochondria associated with these basal folds (Fig. 19.18). They also have considerably fewer and less well developed microvilli than proximal straight tubule cells (compare Figs. 19.15 and 19.18).

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