Step by Step
1
CO₂ is the respiratory acid, controlled by the lungs
CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻. Since the lungs directly control CO₂ levels (through breathing rate), any disorder driven by abnormal CO₂ levels is called a respiratory disorder.
2
HCO₃⁻ is the metabolic base, controlled by the kidneys
Since the kidneys directly control bicarbonate levels, any disorder driven by abnormal bicarbonate levels is called a metabolic disorder.
3
Respiratory disorders
Respiratory acidosis: increased CO₂ (from hypoventilation, COPD, or pneumonia) lowers pH; kidneys compensate by retaining HCO₃⁻. Respiratory alkalosis: decreased CO₂ (from hyperventilation, anxiety, or high altitude) raises pH; kidneys compensate by excreting HCO₃⁻.
4
Metabolic disorders
Metabolic acidosis: decreased HCO₃⁻ (from diabetic ketoacidosis or renal failure) lowers pH; lungs compensate by hyperventilating (Kussmaul breathing) to blow off CO₂. Metabolic alkalosis: increased HCO₃⁻ (from vomiting or antacid overuse) raises pH; lungs compensate by hypoventilating to retain CO₂.
Applied Walkthrough
1
A patient with severe COPD hypoventilates chronically, causing CO₂ to build up in their blood. Since CO₂ is the respiratory acid, this is a respiratory acidosis.
2
Over time, this patient's kidneys compensate by retaining more bicarbonate (HCO₃⁻), helping to buffer the excess acid and bring pH back closer to normal, even though the underlying CO₂ elevation persists.
3
A different patient in diabetic ketoacidosis has depleted bicarbonate levels due to excess metabolic acid production — a metabolic acidosis.
4
This patient's body compensates through the lungs: they begin deep, rapid breathing (Kussmaul breathing) to blow off extra CO₂, helping raise pH back toward normal — illustrating how the respiratory and metabolic systems compensate for each other's primary disorders.
Exam Application
Exams test whether you can identify which of the four acid-base disorders (respiratory acidosis, respiratory alkalosis, metabolic acidosis, metabolic alkalosis) matches a given clinical scenario, and whether you know which organ system (lungs or kidneys) compensates for each.
⚠ Common Trap
The most common trap is confusing which system causes the problem versus which system compensates — remember respiratory disorders are caused by CO₂ imbalances (compensated by the kidneys), while metabolic disorders are caused by HCO₃⁻ imbalances (compensated by the lungs) — the compensating system is always the OTHER one.
✓ Quick Self-Check
1. What ion/molecule is the "respiratory acid," and which organ controls it?
CO₂, controlled by the lungs.
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2. What ion is the "metabolic base," and which organ controls it?
HCO₃⁻ (bicarbonate), controlled by the kidneys.
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3. What causes respiratory acidosis, and how do the kidneys compensate?
Increased CO₂ (from hypoventilation, COPD, pneumonia); kidneys compensate by retaining HCO₃⁻.
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4. What causes metabolic acidosis, and how do the lungs compensate?
Decreased HCO₃⁻ (from diabetic ketoacidosis or renal failure); lungs compensate by hyperventilating (Kussmaul breathing) to blow off CO₂.
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5. What is the normal pH range of blood?
7.35–7.45.
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