30.17
Renal Regulation of Acid-Base Balance
Metabolic reactions in the body produce nonvolatile acids, such as sulfuric acid, which generate an acid load of approximately 1 mEq of H+ per kilogra…
Metabolic reactions within the body generate nonvolatile acids, such as sulfuric acid.
The body tackles this large acid load by regulating the renal reabsorption and secretion of hydrogen and bicarbonate ions.
In the nephron's proximal convoluted tubules, the sodium-hydrogen antiporters secrete hydrogen ions while reabsorbing sodium ions.
In the collecting ducts, two types of intercalated cells regulate acid-base balance through opposing mechanisms.
Type-A intercalated cells secrete hydrogen ions through proton pumps and H+/K+ ATP pumps in their apical membranes, while reabsorbing bicarbonate into the blood via chloride-bicarbonate antiporters in their basolateral membrane.
Type-B intercalated cells function oppositely, with proton pumps in their basolateral membrane and chloride-bicarbonate antiporters in their apical membrane. As a result, bicarbonate ions are secreted, and hydrogen ions are reabsorbed.
The hydrogen ions secreted in the tubular fluid are buffered by combining with monohydrogen phosphate to form dihydrogen phosphate ions and with ammonia to form ammonium ions.
These ions, unable to diffuse back into tubule cells, are excreted in the urine.
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Q1: How do the kidneys handle the acid load produced by metabolic reactions?
Metabolic reactions generate approximately 1 mEq of nonvolatile acids like sulfuric acid per kilogram of body weight daily. The kidneys regulate renal reabsorption and secretion of hydrogen and bicarbonate ions to balance this acid load. Cells in the proximal convoluted tubules and collecting ducts secrete hydrogen ions into the tubular fluid, which are then excreted in urine to maintain pH balance.
Q2: What role do sodium-hydrogen antiporters play in acid-base regulation?
In the proximal convoluted tubules, sodium-hydrogen antiporters secrete hydrogen ions into the tubular fluid while simultaneously reabsorbing sodium ions into the blood. This mechanism helps eliminate excess hydrogen ions from the body while conserving sodium, contributing to both acid-base balance and electrolyte homeostasis.
Q3: How do type A and type B intercalated cells differ in their acid-base functions?
Type A intercalated cells secrete hydrogen ions through proton pumps in their apical membranes and reabsorb bicarbonate via chloride-bicarbonate antiporters in their basolateral membrane, helping excrete excess acid. Type B intercalated cells function oppositely, with proton pumps in their basolateral membrane and chloride-bicarbonate antiporters in their apical membrane, secreting bicarbonate and reabsorbing hydrogen ions when pH is too high.
Q4: What buffering systems prevent hydrogen ions from being reabsorbed in the collecting duct?
Secreted hydrogen ions in the tubular fluid are buffered by combining with monohydrogen phosphate ions to form dihydrogen phosphate and with ammonia to form ammonium ions. These buffered ions cannot diffuse back into tubule cells and are therefore excreted in the urine, preventing hydrogen ion reabsorption and maintaining acid excretion.
Q5: Why are intercalated cells in the collecting duct more important for pH regulation than proximal tubule cells?
While proximal convoluted tubule cells initiate hydrogen ion secretion, intercalated cells in the collecting duct play a more prominent role in regulating body fluid pH. Type A and type B intercalated cells work through opposing mechanisms to fine-tune acid-base balance by responding to pH changes, making them critical for precise pH homeostasis.
Q6: How does the body coordinate renal acid-base regulation with other systems?
The kidneys ensure precise pH regulation through coordinated mechanisms involving hydrogen ion secretion, bicarbonate reabsorption, and buffering in the collecting duct. This renal regulation works alongside respiratory regulation of acid-base balance to safeguard critical physiological processes and maintain stable body pH.
Q7: What determines whether the kidneys excrete or reabsorb bicarbonate?
The type of intercalated cell active in the collecting duct determines bicarbonate handling. Type A intercalated cells reabsorb bicarbonate when pH is low, while type B intercalated cells secrete bicarbonate when pH is high. This opposing mechanism allows the kidneys to adjust bicarbonate levels dynamically in response to body pH changes.