25.6 Tubular reabsorption  (Page 7/18)

At the transition from the DCT to the collecting duct, about 20 percent of the original water is still present and about 10 percent of the sodium. If no other mechanism for water reabsorption existed, about 20–25 liters of urine would be produced. Now consider what is happening in the adjacent capillaries, the vasa recta. They are recovering both solutes and water at a rate that preserves the countercurrent multiplier system. In general, blood flows slowly in capillaries to allow time for exchange of nutrients and wastes. In the vasa recta particularly, this rate of flow is important for two additional reasons. The flow must be slow to allow blood cells to lose and regain water without either crenating or bursting. Second, a rapid flow would remove too much Na ⁺ and urea, destroying the osmolar gradient that is necessary for the recovery of solutes and water. Thus, by flowing slowly to preserve the countercurrent mechanism, as the vasa recta descend, Na ⁺ and urea are freely able to enter the capillary, while water freely leaves; as they ascend, Na ⁺ and urea are secreted into the surrounding medulla, while water reenters and is removed.

Reabsorption and secretion in the distal convoluted tubule

Approximately 80 percent of filtered water has been recovered by the time the dilute forming urine enters the DCT. The DCT will recover another 10–15 percent before the forming urine enters the collecting ducts. Aldosterone increases the amount of Na ⁺ /K ⁺ ATPase in the basal membrane of the DCT and collecting duct. The movement of Na ⁺ out of the lumen of the collecting duct creates a negative charge that promotes the movement of Cl ^– out of the lumen into the interstitial space by a paracellular route across tight junctions. Peritubular capillaries receive the solutes and water, returning them to the circulation.

Cells of the DCT also recover Ca ⁺⁺ from the filtrate. Receptors for parathyroid hormone (PTH) are found in DCT cells and when bound to PTH, induce the insertion of calcium channels on their luminal surface. The channels enhance Ca ⁺⁺ recovery from the forming urine. In addition, as Na ⁺ is pumped out of the cell, the resulting electrochemical gradient attracts Ca ⁺⁺ into the cell. Finally, calcitriol (1,25 dihydroxyvitamin D, the active form of vitamin D) is very important for calcium recovery. It induces the production of calcium-binding proteins that transport Ca ⁺⁺ into the cell. These binding proteins are also important for the movement of calcium inside the cell and aid in exocytosis of calcium across the basolateral membrane. Any Ca ⁺⁺ not reabsorbed at this point is lost in the urine.

Collecting ducts and recovery of water

Solutes move across the membranes of the collecting ducts, which contain two distinct cell types, principal cells and intercalated cells. A principal cell    possesses channels for the recovery or loss of sodium and potassium. An intercalated cell    secretes or absorbs acid or bicarbonate. As in other portions of the nephron, there is an array of micromachines (pumps and channels) on display in the membranes of these cells.