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[Blood]
           |
    +--------------+
    |              |
  H₂CO₃ (Weak Acid)  + H⁺ (from added acid) → HCO₃⁻ (Conjugate Base)
    |              
    |              |
  HCO₃⁻ (Conjugate Base)  + OH⁻ (from added base) → H₂CO₃ (Weak Acid)
    +--------------+