Reabsorption of Nutrients p. 5262. Symport carriers in the proximal tubule transfer both Na+ and the organic molecule from the lumen into the cell (Na+ passively; organics actively via symport carrier). 3. Glucose and amino acids passively diffuse down their concentration gradients across the basolateral membrane through the interstitial fluid into the capillary plasma.A. Cl- passively reabsorbed down the electrical gradient created by the Na+ entry into the peritubular cell; Cl-passes between, not into the tubular cells and into the peritubular capillaries following Na+.4. Not only is secondary active reabsorption of glucose and amino acids linked to the basolateral Na+/K+ pump, but passive reabsorption of:(A) Cl-, (B) H2O (Fig. 14-19 ) and (C) urea (Fig. 14-20) also depends on this active Na+ reabsorption mechanism1. Glucose[NOT REGULATED BY KIDNEY; normally regulated by endocrine and liver mechanisms] and amino acids are reabsorbed by Na+-dependent secondary active transport.
Tubular cellInterstitial fluidPeritubularcapillaryLumenBasolateralNa+– K+ pumpLateral spaceKEY= Active transport of ion against concentration gradient= Passive movement of ion down concentration gradientGlucose, AAs, water & urea p. 526DiffusionCl--PROXIMAL Na+ & glucose cotransporter (SGLT)Active TPassivelyp. 524p. 524
Reabsorption- WaterB. Water is passively reabsorbed throughout the tubule as it follows reabsorbed Na+. :a. 80% of the water reabsorbed is uncontrolledb. 65% is reabsorbed in the proximal tubule where Aquaporin (AQP-1) channels are always open (see next slide).c. 15% is reabsorbed from the loop of Henled. 20% of the water reabsorbed is controlled under hormonal control of vasopressin (ADH) at the distal tubule where Aquaporin (AQP-2) channels are located
AQP-1waterchannelsInterstitial fluidPeritubularcapillaryLumenProximal tubular cellOsmosisHydrostaticpressureAQP-1 waterchannelsOsmosisFig. 14-19, p. 527“Leaky” tight junctionsHypertonic regions in the lateral spaces (osmotic effects) Always open:Always open: