Acid-Base Physiology and ABG Interpretation
Core buffers, renal acid handling, and a systematic five-step approach to reading arterial blood gases.
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Questions Covered in This Set
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Normal arterial pH range and corresponding [H⁺]
pH 7.35–7.45; [H⁺] ≈ 40 nanomolar at pH 7.4 (about a millionth the concentration of sodium).
Write the Henderson–Hasselbalch equation for the CO₂/bicarbonate system
pH = 6.1 + log([HCO₃⁻] / (0.03 × PCO₂)). Normal: 6.1 + log(24/1.2) = 6.1 + 1.3 = 7.4.
Why is bicarbonate a great buffer despite a pKa of 6.1?
It is an OPEN system: lungs blow off CO₂ without limit and kidneys regenerate HCO₃⁻ without limit, so the buffer is never exhausted.
Kassirer–Bleich (memory) formula
[H⁺] (nM) ≈ 24 × PCO₂ / [HCO₃⁻]. At pH 7.4, [H⁺] = 40 nM; add ~1 nM for each 0.01 pH unit below 7.4.
Three lines of pH defense and their timescales
Chemical buffers (seconds), respiratory compensation (minutes), renal compensation (hours to days).
Daily acid loads the body must handle
~15,000 mmol/day volatile acid (CO₂) from metabolism, plus 50–100 mEq/day fixed acid (sulfuric from methionine/cysteine, phosphoric from phospholipids).
Kidney Job 1: bicarbonate reclamation — where and how?
80–90% in the proximal tubule via NHE3; luminal carbonic anhydrase IV splits H₂CO₃, CA II recombines intracellularly, HCO₃⁻ exits via Na⁺/3HCO₃⁻ cotransporter. Reclaims existing base — makes no new base. Blocked by acetazolamide.
Kidney Job 2: generating NEW bicarbonate
α-intercalated cell H⁺-ATPase secretes H⁺ onto a urinary buffer; each buffered H⁺ excreted yields one new HCO₃⁻ into blood via basolateral Cl⁻/HCO₃⁻ exchanger (AE1).
The two urinary buffers and their relative contributions
Phosphate (titratable acid, ~1/3, pKa 6.8) and ammonium (~2/3). Free H⁺ alone only takes urine to pH ~4.5, so buffers are essential.
Why is renal compensation slow?
It depends on upregulating glutamine ammoniagenesis in the proximal tubule; NH₄⁺ excretion rises several-fold only over days, unlike near-instant ventilatory change.
First three steps of the five-step ABG approach
1) Acidemia (pH<7.35) or alkalemia (pH>7.45)? 2) Which side drives it — the primary disturbance moves WITH the pH. 3) Is compensation appropriate? Compensation never fully corrects pH; over/undershoot means a second disorder.
Role of hemoglobin and bone as buffers
Hemoglobin is the main intracellular blood buffer (isohydric shift lets venous blood carry CO₂); bone carbonate dissolves in chronic acidosis, explaining bone disease in CKD and RTA.
