Chapter 4 - Chapter 4 Renal Transport Mechanisms: NaCl and...

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Chapter 4 Renal Transport Mechanisms: NaCl and H2O Absorption along the Nephron Basic Transport Mechanisms for Reabsorption 1. Passive Transport/Diffusion Diffusion to (down chemical concentration gradient) Develops spontaneously No metabolic energy needed Cations (e.g., Na+, K+) move to negative side of membrane, Anions (e.g., Cl-, HCO3-) move to positive side of membrane by diffusion Lipid-soluble substances (e.g., gasses, O2, CO2, and NH3) through lipid bilayer of membrane occurs through diffusion Osmosis Osmosis of water occurs through channels (pores) in the membrane. - Driven by osmotic pressure gradients Solvent Drag When water reabsorbed across tubule segments, solutes dissolved in water also carried along with water – solvent drag - Accounts for substantial solute reabsorption across proximal tubule Facilitated Diffusion Uniport Carrier-mediated, in one direction Urea and glucose movement 2. Active Processes Carrier required Against electrochemical gradient Requires energy - direct: 1 ° A.T. - indirect: 2 ° A.T. I. Coupled Transport A. Symport Na+-glucose Na+-amino acid [proximal tubule] Na+-phosphate
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1Na+-1K+-2Cl- [thick ascending limb of Henle’s loop] B. Antiport Na+-H+ antiporter in proximal tubule (apical membrane) mediates Na+ reabsorption and H+ secretion - Uphill movement of H+ out of the cell into tubular lumen and driven by movement of Na+ from tubular into cells down the Na+ gradient - This is an example of ???? 2 ° active transport Na+/K+ ATPase - located in basolateral membrane H+-ATPase [responsible for H+ secretion in H+-K+ ATPase collecting duct system] Ca+ ATPase – Ca2+ movement from cytoplasm to blood II. Endocytosis Invagination of C.M, engulfing, and pinchind off an endocytic vesicle. Reabsorption of small proteins and MM by proximal tubule Requires ATP General Principles of Transepithelial Solute and H2O Transport Reabsorption Net transport of a substance from - Tubular lumen into the blood Secretion Net transport of a substance from the - Blood into tubular lumen Tight Junctions between Tubular Cells Renal cells are held together by tight junctions A. Paracellular Pathway Reabsorption or secretion of substances between cells Example: - Reabsorption of Ca+ and K+ across proximal tubule via solvent drag B. Transcellular pathway Reabsorption or secretion of substances through cells. Example: - Reabsorption of Na+ by proximal tubule.
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- Two step process: a) Movement across the apical membrane into the cell, down electrochemical gradient established by Na+-K+ ATPase pump. b) Movement across basolateral membrane against electrochemical gradient via the Na+-K+ ATPase pump. NaCl and H2O Reabsorption along the Nephron
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This note was uploaded on 01/07/2012 for the course PHCH 6234 taught by Professor Farqui during the Winter '11 term at Palmer Chiropractic.

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Chapter 4 - Chapter 4 Renal Transport Mechanisms: NaCl and...

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