Maintaining acid-base balance is vital for cellular function. Our bodies tightly regulate blood pH around 7.4, since even slight deviations can alter enzyme activity and protein structure. Let’s break down how clinicians approach acid-base disorders in a systematic way.
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⚗️ The Physiology: The Buffer System
The acid-base balance in the body is determined by the carbonic acid–bicarbonate buffer system:
H₂O + CO₂ ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
- Respiratory system: controls CO₂ levels — compensation is rapid (within 12–24 hours).
- Renal system: controls bicarbonate (HCO₃⁻) — compensation is slower (up to 5–6 days).
- The Henderson-Hasselbalch equation links these variables: [H+]=24×pCO2[HCO3−][H^+] = 24 \times \frac{pCO₂}{[HCO₃⁻]}[H+]=24×[HCO3−]pCO2 pH depends on the ratio of pCO₂ (acid component) to HCO₃⁻ (base component).
🧠 Stepwise Clinical Approach
Step 1: Acidosis or Alkalosis?
Check the pH:
- pH < 7.4 → Acidemia
- pH > 7.4 → Alkalemia
This determines whether the primary disturbance is acidotic or alkalotic.
Step 2: Identify the Origin — Respiratory or Metabolic
- ↓ HCO₃⁻ → Metabolic Acidosis
- ↑ pCO₂ → Respiratory Acidosis
- ↑ HCO₃⁻ → Metabolic Alkalosis
- ↓ pCO₂ → Respiratory Alkalosis
Step 3: Determine Compensation — Acute or Chronic
Assess how the opposite system compensates:
- Metabolic Acidosis → ↓ pCO₂ (respiratory compensation)
- Respiratory Acidosis → ↑ HCO₃⁻ (renal compensation)
- Metabolic Alkalosis → ↑ pCO₂
- Respiratory Alkalosis → ↓ HCO₃⁻
Compensation magnitude tells you if the disorder is acute or chronic.
Step 4: Check for an Anion Gap (AG)
AG=[Na+]−([Cl−]+[HCO3−])AG = [Na⁺] – ([Cl⁻] + [HCO₃⁻])AG=[Na+]−([Cl−]+[HCO3−])
- AG > 10 → Anion Gap Metabolic Acidosis (e.g., DKA, lactic acidosis)
- AG ≤ 10 → Non–anion gap metabolic acidosis (e.g., diarrhea, RTA)
Step 5: Calculate Delta–Delta
Used to detect mixed disorders: ΔΔ=(CalculatedAG−ExpectedAG)/(NormalHCO3−−MeasuredHCO3−)ΔΔ = (Calculated AG – Expected AG) / (Normal HCO₃⁻ – Measured HCO₃⁻)ΔΔ=(CalculatedAG−ExpectedAG)/(NormalHCO3−−MeasuredHCO3−)
- > 2 → AGMA + Metabolic Alkalosis
- 1–2 → Pure AGMA
- < 1 → AGMA + Non-AG Metabolic Acidosis
🩸 Clinical Takeaway
Understanding acid-base balance isn’t just about lab numbers — it’s about recognizing patterns and compensations. Using this structured approach helps clinicians identify primary disturbances, compensatory responses, and mixed disorders accurately, guiding appropriate treatment.