The reasoning: Henderson–Hasselbalch and three lines of defence
pH = 6.1 + log([HCO₃⁻] / (0.03 × PaCO₂)) — at HCO₃⁻ 24, PaCO₂ 40: 6.1 + log(24/1.2) = 6.1 + 1.3 = 7.4. The bicarbonate system is powerful because it is open (CO₂ removed by lungs, HCO₃⁻ regulated by kidneys), despite a pKa (6.1) far from 7.4. Other buffers: haemoglobin (major intracellular), plasma proteins, phosphate (intracellular, urinary), bone carbonate (chronic acidosis → bone loss). Isohydric principle: all buffer pairs equilibrate with the same [H⁺].