BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

20.5. The Human Kidney

20.5.7. The Distal Convoluted Tubule and Collecting Duct

The final two sections of the nephron—the distal convoluted tubule and the collecting duct—fine-tune the composition of Body Fluids. The proximal convoluted tubule always acts uniformly, reabsorbing a constant proportion of Water and salts via the mechanism described in Section 20.5.5. It provides rough homeostatic control, ensuring the reabsorption of the bulk of essential substances back into the bloodstream.

Osmoregulation requires precise control over The amount of water and salts reabsorbed, adjusted to meet the body's overall physiological needs. This task is carried out by the distal convoluted tubule and collecting duct, which also participate in regulating Blood pH. These Functions are discussed in detail in Sections 20.6 and 20.7.

The Cells of the distal tubule share a similar Structure with those of the proximal tubule: their luminal surface is lined with microvilli to increase the surface area for reabsorption, and they are packed with Cell/35.html">Mitochondria to fuel Active Transport. The collecting duct carries fluid from the outer (cortical) zone of the renal medulla toward the renal papilla (Fig. 20.12). As it descends, the surrounding tissue fluid becomes increasingly concentrated due to the countercurrent mechanism described above. Consequently, water is drawn out of the collecting duct by osmosis, and the final concentration of the urine can theoretically match that of the medullary tissue—approximately 1200. In reality, the actual volume of water lost here is regulated by antidiuretic hormone via the mechanism examined in Section 20.6.

Table 20.4 compares the composition of Blood Plasma and urine. It is evident that urine exhibits higher concentrations of all dissolved solutes except sodium ions, while Proteins and glucose are normally absent.

Class="center">Table 20.4. Composition of blood plasma and urine, and the concentration changes of solutes during Urine Formation in humans