25.6 Tubular reabsorption  (Page 4/18)

About 67 percent of the water, Na ⁺ , and K ⁺ entering the nephron is reabsorbed in the PCT and returned to the circulation. Almost 100 percent of glucose, amino acids, and other organic substances such as vitamins are normally recovered here. Some glucose may appear in the urine if circulating glucose levels are high enough that all the glucose transporters in the PCT are saturated, so that their capacity to move glucose is exceeded (transport maximum, or T _m ). In men, the maximum amount of glucose that can be recovered is about 375 mg/min, whereas in women, it is about 300 mg/min. This recovery rate translates to an arterial concentration of about 200 mg/dL. Though an exceptionally high sugar intake might cause sugar to appear briefly in the urine, the appearance of glycosuria    usually points to type I or II diabetes mellitus. The transport of glucose from the lumen of the PCT to the interstitial space is similar to the way it is absorbed by the small intestine. Both glucose and Na ⁺ bind simultaneously to the same symport proteins on the apical surface of the cell to be transported in the same direction, toward the interstitial space. Sodium moves down its electrochemical and concentration gradient into the cell and takes glucose with it. Na ⁺ is then actively pumped out of the cell at the basal surface of the cell into the interstitial space. Glucose leaves the cell to enter the interstitial space by facilitated diffusion. The energy to move glucose comes from the Na ⁺ /K ⁺ ATPase that pumps Na ⁺ out of the cell on the basal surface. Fifty percent of Cl ^– and variable quantities of Ca ⁺⁺ , Mg ⁺⁺ , and {\text{HPO}}_4^{2-} are also recovered in the PCT.

Recovery of bicarbonate (HCO ₃ ^– ) is vital to the maintenance of acid–base balance, since it is a very powerful and fast-acting buffer. An important enzyme is used to catalyze this mechanism: carbonic anhydrase (CA). This same enzyme and reaction is used in red blood cells in the transportation of CO ₂ , in the stomach to produce hydrochloric acid, and in the pancreas to produce HCO ₃ ^– to buffer acidic chyme from the stomach. In the kidney, most of the CA is located within the cell, but a small amount is bound to the brush border of the membrane on the apical surface of the cell. In the lumen of the PCT, HCO ₃ ^– combines with hydrogen ions to form carbonic acid (H ₂ CO ₃ ). This is enzymatically catalyzed into CO ₂ and water, which diffuse across the apical membrane into the cell. Water can move osmotically across the lipid bilayer membrane due to the presence of aquaporin water channels. Inside the cell, the reverse reaction occurs to produce bicarbonate ions (HCO ₃ ^– ). These bicarbonate ions are cotransported with Na ⁺ across the basal membrane to the interstitial space around the PCT ( [link] ). At the same time this is occurring, a Na ⁺ /H ⁺ antiporter excretes H ⁺ into the lumen, while it recovers Na ⁺ . Note how the hydrogen ion is recycled so that bicarbonate can be recovered. Also, note that a Na ⁺ gradient is created by the Na ⁺ /K ⁺ pump.