Renal bicarbonate handling

Bicarbonate Reabsorption - The Recycling Plant

The Proximal Convoluted Tubule (PCT) reclaims ~85% of filtered bicarbonate, preventing its loss in urine. This process is indirect, relying on proton secretion and carbon dioxide conversion, not direct transport of bicarbonate from the lumen.

• Lumen: Filtered $HCO_3^- + H^+ \rightarrow H_2CO_3$, which is then converted to $H_2O + CO_2$ by luminal carbonic anhydrase.
• PCT Cell:
  • $CO_2$ and $H_2O$ diffuse into the cell.
  • Intracellular carbonic anhydrase reverses the reaction: $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$.
  • $H^+$ is secreted into the lumen via the Na+/H+ exchanger (NHE3).
  • $HCO_3^-$ is transported into the blood via the basolateral Na+/HCO3- cotransporter (NBCe1).

⭐ Carbonic anhydrase inhibitors (e.g., acetazolamide) cause a self-limited bicarbonate diuresis by blocking this primary reabsorption mechanism.

New Bicarb Generation - Making Fresh Buffer

• Location: α-intercalated cells of the collecting duct.

• Function: Excretes acid (H+) and generates new bicarbonate to combat acidosis, unlike reabsorption which just reclaims filtered bicarb.

• Mechanisms of H+ Excretion & New HCO3- Generation:

  • 1. Titratable Acid Formation:

    • H+ is secreted into the lumen via H+-ATPase and H+/K+-ATPase.
    • Secreted H+ binds to urinary phosphate: $HPO_4^{2-} + H^+ \rightarrow H_2PO_4^-$.
    • For each H+ excreted, one new $HCO_3^-$ is generated and moves to the blood.

  • 2. Ammoniagenesis (Glutamine Metabolism):

    • Proximal tubule cells metabolize glutamine: $Glutamine \rightarrow 2NH_4^+ + 2HCO_3^-$.
    • NH3 diffuses into the collecting duct lumen, where it's 'trapped' by H+ to become NH4+, which is then excreted.

⭐ In chronic acidosis, the synthesis and excretion of ammonium (NH4+) becomes the predominant mechanism for excreting acid and generating new bicarbonate.

Regulation of Handling - The Control Knobs

• Factors ↑ HCO₃⁻ Reabsorption:

  • ↑ Arterial PCO₂ (Respiratory Acidosis)
  • ↓ ECF volume & ↑ Angiotensin II
  • ↓ Plasma [K⁺] (Hypokalemia)

• Factors ↓ HCO₃⁻ Reabsorption:

  • ↓ Arterial PCO₂ (Respiratory Alkalosis)
  • ↑ ECF volume & ↓ Angiotensin II
  • ↑ Plasma [K⁺] (Hyperkalemia)
  • Parathyroid Hormone (PTH)

⭐ Angiotensin II stimulates the Na⁺/H⁺ exchanger (NHE3) in the PCT, increasing H⁺ secretion and thus increasing bicarbonate reabsorption. This is a key mechanism behind contraction alkalosis.

📌 Mnemonic: Think 'Contraction Alkalosis' for the combined effect of low ECF volume and high Angiotensin II.

High-Yield Points - ⚡ Biggest Takeaways

• The proximal convoluted tubule (PCT) reclaims ~85% of filtered bicarbonate (HCO₃⁻), a process driven by luminal carbonic anhydrase.
• H⁺ secretion into the tubular fluid, primarily via the Na⁺/H⁺ exchanger (NHE3), is the rate-limiting step for HCO₃⁻ reabsorption.
• α-intercalated cells in the collecting duct generate "new" HCO₃⁻ while actively secreting H⁺ via H⁺-ATPase, which is critical during acidosis.
• β-intercalated cells perform the opposite, secreting HCO₃⁻ during alkalosis.
• Net acid excretion as titratable acids (e.g., H₂PO₄⁻) and ammonium (NH₄⁺) corresponds to net HCO₃⁻ generation.