Renal Phys 9

• intracellular fluid [K+] =

  150 mM - controlled by Na+/K+-ATPase in cells
• extracellular fluid [K+] =

  5 mM - controlled by kidneys
• this can cause lethal malfunction of excitable tissues (nerves and muscles)


  small changes in ECF [K+]
• hypokalemia

  low K+ in ECF
• hyperkalemia

  high K+ in ECF
• potassium APs in healthy and diseased individuals


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• K+ experiences all three basic renal processes: filtration, reabsorption, secretion


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• K+ is reabsorbed paracellularly by following H2O, which followed Na+ (all by following H -> L diffusion gradients)


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• in the proximal convoluted tubule, there is net K+ reabsorption from this tubular segment
• 65% of K+ reabsorption occurs here is non-regulated (same as for Na+, Cl-, and H2O); intrinsic
• in the ascending loop of Henle, there is net K+ reabsorption from this tubular segment


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• in the cortical collecting duct, there is net K+ secretion from this tubular segment

  • aldosterone regulates K+ secretion by building pumps (on basolateral membrane) and channels (on apical membrane)


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• aldosterone control of Na+ reabsorption and K+ secretion


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• major functions: ultrafiltrate of plasma

  controlling factors: starling factors (P GC, P BS, pi GC)


  Glomerulus/Bowman's capsule
• major functions: bulk reabsorption of solutes and water; secretion of solutes (except K+) and organic acids and bases

  controlling factors: active transport of solutes with passive water reabsorption


  proximal tubule
• establishes medullary osmotic gradient (juxtamedullary nephrons)

  secretion of urea


  loop of Henle
• major function: bulk reabsorption of water

  controlling factor: passive water reabsorption


  descending limb of Henle
• major function: reabsorption of Na+ and Cl-

  controlling factor: active transport driving reabsorption by cotransport


  ascending limb of Henle
• major functions: fine-tuning of the reabsorption/secretion of small quantities of useful solutes remaining

  controlling factors: aldosterone stimulates Na+ reabsorption and K+ secretion


  cortical collecting duct
• major functions: fine-tuning of water reabsorption and reabsorption of urea

  controlling factor: vasopressin increases passive reabsorption of water


  cortical and medullary collecting ducts