Acid-Base Regulation by the Kidneys

Renal Acid-Base: Basics - Bicarb's Big Save

• Kidneys: Long-term acid-base balance; slower than lungs.
• Primary role: Reclaims filtered $HCO_3^-$ (major buffer), preventing its urinary loss.
• $HCO_3^-$ Reabsorption:
  • ~80-90% in Proximal Convoluted Tubule (PCT).
  • Rest in Thick Ascending Limb (TAL), Distal Tubule (DT), Collecting Ducts (CD).
  • Indirect mechanism:
    • Luminal: $H^+ + HCO_3^- \rightarrow H_2CO_3; H_2CO_3 \xrightarrow{CA_{IV}} CO_2 + H_2O$.
    • Cellular: $CO_2 + H_2O \xrightarrow{CA_{II}} H_2CO_3 \rightarrow H^+ + HCO_3^-$.
    • $HCO_3^-$ exits to blood (e.g., NBCe1-A in PCT).
    • $H^+$ secreted to lumen (NHE3, H-ATPase).
• Key Enzymes: Carbonic Anhydrase (CA-IV luminal, CA-II cytoplasmic).

⭐ Majority of $HCO_3^-$ reabsorption occurs in the Proximal Convoluted Tubule (PCT), approximately 80-90%.

Renal Acid-Base: Acid Out - Proton Pump & Buffers

• Kidneys excrete 50-100 mEq/day of non-volatile acids, generating new $HCO_3^-$.
• H⁺ Secretion Mechanisms (Apical, Type A Intercalated Cells):
  • H⁺-ATPase (Proton Pump): Directly secretes H⁺.
  • H⁺-K⁺-ATPase: Secretes H⁺, reabsorbs K⁺.
• Urinary Buffers (combine with secreted H⁺ in lumen):
  • Phosphate Buffer: $HPO_4^{2-} + H^+ \rightleftharpoons H_2PO_4^-$ (Titratable Acid). pKa 6.8.
  • Ammonia Buffer: $NH_3 + H^+ \rightleftharpoons NH_4^+$.
    • $NH_3$ from glutamine metabolism (PCT). 📌 Glutamine gives an amine.
    • $NH_3$ diffuses into lumen, combines with H⁺ → $NH_4^+$ (trapped).
    • Major adaptive response to acidosis.

    ⭐ Glutamine is the primary source of $NH_4^+$ produced by the kidneys, crucial for excreting large acid loads.

• Net Acid Excretion (NAE):
  • NAE = (Titratable Acid + $NH_4^+$) - (Excreted $HCO_3^-$)
  • Represents net acid eliminated & new $HCO_3^-$ gained by blood.

Renal Acid-Base: Control Knobs - Modulating Factors

Renal acid-base handling is modulated by:

• Arterial $PCO_2$:
  • ↑$PCO_2$ → ↑H+ secretion, ↑$HCO_3^-$ reabsorption.
  • ↓$PCO_2$ → ↓H+ secretion, ↓$HCO_3^-$ reabsorption.
• Potassium (K+):
  • Hypokalemia → ↑H+ secretion, ↑$HCO_3^-$ reabsorption.
  • Hyperkalemia → ↓H+ secretion, ↓$HCO_3^-$ reabsorption.
• Hormones:
  • Aldosterone: ↑H+ secretion.
  • Angiotensin II: ↑H+ secretion, ↑$HCO_3^-$ reabsorption (via NHE3).
• ECF Volume:
  • Depletion → ↑H+ secretion, ↑$HCO_3^-$ reabsorption (via RAAS).
  • Expansion → Opposite.

⭐ Hypokalemia stimulates H+ secretion and bicarbonate reabsorption, potentially leading to metabolic alkalosis.

Renal Acid-Base: Imbalance Response - Kidney to Rescue

Kidneys combat systemic acid-base disturbances by modulating three key processes: $H^+$ secretion, $HCO_3^-$ reabsorption, and $HCO_3^-$ generation (or enhanced excretion in alkalosis).

High‑Yield Points - ⚡ Biggest Takeaways

• Kidneys manage acid-base via H+ secretion, HCO3- reabsorption, and new HCO3- generation.
• PCT reabsorbs ~85% filtered HCO3-, aided by carbonic anhydrase.
• H+ secretion occurs in PCT, TAL, and collecting ducts (CD) via H+-ATPase & H+-K+-ATPase.
• Excretion of titratable acids (e.g., H2PO4-) and NH4+ eliminates fixed acids, regenerates HCO3-.
• Acidosis: ↑ H+ secretion, ↑ NH4+ production, ↑ HCO3- reabsorption.
• Alkalosis: ↓ H+ secretion, ↓ NH4+ production, ↑ HCO3- excretion.