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 movement2.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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