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 hyposmotic. In addition, epithelial cells lining the thick ascending limb produce an 85-kilodalton protein called uromodulin (Tamm-Horsfall protein) that influences NaCl reabsorption and urinary concentration ability. Uromodulin also modulates cell adhesion and signal transduction by interacting with various cytokines, as well as it inhibits the aggregation of calcium oxalate crystals (preventing kidney stones formation) and provides a defense against urinary tract infection. In individuals with inﬂammatory kidney diseases, a precipitated uromodulin is detected in urine in the form of urinary casts (see Folder 20.3).
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 Henleand 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 Cl , Na, and K to enter the cell from the lumen. Na is actively transported across the extensive basolateral plications by the Na/K-ATPase pumps; Cl and K diffuse out from the intracellular space by the Cl and K channels. Some K ions leak back into the tubular fluid through K channels, causing the tubular lumen to be positively charged with respect to the interstitium. This positive gradient provides the driving force for the reabsorption 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 (Plate 77, page 734). 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 basolateral plications, and there are numerous mitochondria associated with these basal folds (Fig. 20.21). They also have considerably fewer and less well-developed microvilli than proximal straight tubule cells (compare Figs. 20.18 and 20.19).
The distal convoluted tubule exchanges Na for K under aldosterone regulation.
The distal convoluted tubule, located in the cortical labyrinth, is only about one third as long (5 mm) as the proximal convoluted tubule. This short tubule is responsible for  reabsorption of Naand secretion of K into the ultrafiltrate to conserve Na .
FIGURE 20.21 • Electron micrograph of a distal convoluted tubule cell. The apical surface of the cell displays some microvilli (Mv), but they are not sufficiently long or numerous to give the appearance of a brush border (compare with Fig. 20.15). The nucleus and Golgi apparatus (G) are in the upper portion of the cell. Mitochondria (M) are chiefly in the basal region of the cell within the interdigitating processes (IP). As in the proximal tubule cell, the mitochondria account for the appearance of basal striations in the light microscope. A basal lamina (BL) is seen adjacent to the basal surface of the cell. 12,000.