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. 2020 Dec 26;38(2):949–968. doi: 10.1007/s12325-020-01587-5

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© The Author(s) 2020

Open AccessThis article is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, which permits any non-commercial use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc/4.0/.

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Fig. 1 — Schematic diagrams illustrating bicarbonate (HCO₃^–) reabsorption and regeneration in the kidney. a HCO₃^– reabsorption in the proximal tubule. Hydrogen ions (H⁺) are secreted into the lumen via apical sodium (Na⁺)/H⁺ exchanger 3 (NHE3) and H⁺-ATPase transporters. Apical carbonic anhydrase (CA) IV catalyzes the reaction between H⁺ and HCO₃^–, which forms H₂CO₃ that rapidly dissociates to water and carbon dioxide (CO₂). CO₂ diffuses back across the apical membrane, where CA-II catalyzes its reaction with intracellular hydroxide ions (OH^–) to form H⁺ and HCO₃^–. HCO₃^– is transported across the basolateral membrane by the Na⁺/HCO₃^–/CO₃^2– cotransporter (NBCe1). b HCO₃^– reabsorption in the thick ascending limb. As in the proximal tubule, H⁺ is secreted into the lumen via NHE3, where it reacts with HCO₃^– to release CO₂ that diffuses back across the apical membrane. HCO₃^– is transported across the basolateral membrane by NBCe1 and the kidney anion exchanger (AE1). c H⁺ secretion by α-intercalated cells in cortical collecting duct (CCD). H⁺ is secreted into the lumen by H⁺/K⁺-ATPase and vacuolar (v) H⁺-ATPase transporters on the apical membrane. Intracellular OH^– generated by H⁺ secretion reacts with CO₂ via CA-II to form HCO₃^–, which is removed by basolateral AE1. The resulting intracellular chloride (Cl^–) exits via conductance channels in the basolateral membrane. Luminal K⁺ transported into the cell via H⁺/K⁺-ATPase can exit via channels in the apical or basolateral membrane, depending on K⁺ balance. d HCO₃^– secretion by β-intercalated cells in CCD. H⁺-ATPase transports H⁺ across the basolateral membrane. Intracellular OH^– generated by H⁺ secretion reacts with CO₂ via CA-II to form HCO₃^–, which is transported into the lumen by the apical Cl^–/HCO₃^– exchanger pendrin. Intracellular Cl^– exits via conductance channels in the basolateral membrane

(These figures were published in Comprehensive Clinical Nephrology: 5th Edition, Palmer BF, Normal acid–base balance, pp. 142–148, Copyright Elsevier (2014) [6])