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Acid-Base Balance (Buffers, Lungs & Kidneys Together Maintain Acid…
Acid-Base Balance
- Buffers, Lungs & Kidneys Together Maintain Acid-Base Balance
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Titrate H+ including hemoglobin, other proteins, carbonate in bone, phosphate & bicarbonate.
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- Acid Excretion Is Achieved by Proton Secretion by Tubule Epithelial Cells, Buffering in the Tubule Fluid, & Bicarbonate Absorption
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Proximal tubule: bicarbonate reabsorption mediated by sodium bicarbonate co-transporter,NBCel.
Collecting duct: basolateral anion exchanger, kAE1, is a chloride/bicarbonate exchanger.
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- The Collecting Duct Can Secrete Bicarbonate & Generate Alkaline Urine
Proximal tubule reabsorbs bicarbonate ions & secrete H+, regardless of plasma bicarbonate ion concentration and blood pH.
Plasma bicarbonate ions concentration increases, concentration of bicarbonate ion in glomerular filtrate increase & amount of bicarbonate ion reabsorption by proximal tubule epithelium increases.
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Cl- depletion, amount of Cl- delivered to collecting duct fall so low that bicarbonate secretion is impaired because inadequate luminal Cl- is available for exchange with intracellular bicarbonate ions.
- Renal Ammonia Metabolism Generates New Bicarbonate & Promotes Acid Excretion
Ammoniagenesis: In proximal tubule cells amino acid glutamine is metabolized to produce ammonium ion.
Thick ascending limb of Henle's loop: luminal ammonium ion is reabsorbed by substitution for K+ on apical Na+,K+,2Cl- co-transporter.
Ammoniagenesis & ammonia excretion to control acid-base homeostasis in mice, rats, dogs, chickens & humans.
- The Collecting Duct Can Secrete Protons, Reabsorb Bicarbonate, & Generate Acidic Urine
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Intercalated cells: have carbonic anhydrase, which catalyzes formation of intracellular H+ & bicarbonate ion from intracellular water & carbon dioxide.
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- The Proximal Tubule Has a High Capacity for H+ Secretion & Bicarbonate Reabsorption
Apical plasma membrane-bound carbonic anhydrase catalyzes the formation of water and carbon dioxide from filtered bicarbonate ion & secreted H+. Carbon dioxide enters epithelial cell & combines with intracellular water to form bicarbonate ions and H+ catalyzed by cytoplasmic carbonic anhydrase.
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- The Thick Ascending Limb of Henle's Loop Absorbs Filtered Bicarbonate
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- The Collecting Duct Determines the Final Urine pH
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