Respiratory System and Carbon Dioxide Elimination: Mechanisms and Transport

CO₂ Removal: A Multi-Step Process

1. Pulmonary Gas Exchange

In alveoli, CO₂ diffuses from pulmonary capillary blood (PCO₂ ~45 mmHg) into alveolar air (PCO₂ ~40 mmHg) down its partial pressure gradient. Simultaneously, O₂ enters blood.

2. Ventilation Control

Central chemoreceptors (medulla) respond to CSF pH (driven by arterial PCO₂). Peripheral chemoreceptors (carotid/aortic bodies) respond to PaO₂, PaCO₂, pH. ↑PCO₂ → ↑ventilation (rate × tidal volume).

3. CO₂ Transport in Blood (Three Forms)

• Dissolved (10%): CO₂ physically dissolved in plasma.
• Carbaminohemoglobin (20–30%): CO₂ binds to hemoglobin amino termini.
• Bicarbonate (60–70%): CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (catalyzed by carbonic anhydrase in RBCs). HCO₃⁻ exits RBC in exchange for Cl⁻ (Hamburger shift). H⁺ buffered by hemoglobin.

4. Exhalation

In lungs, reactions reverse: HCO₃⁻ re-enters RBC, combines with H⁺ → H₂CO₃ → CO₂ + H₂O → CO₂ diffuses into alveoli → exhaled.

Clinical Relevance

• Hypoventilation → respiratory acidosis (↑PCO₂, ↓pH).
• Hyperventilation → respiratory alkalosis (↓PCO₂, ↑pH).
• CO₂ transport impaired in anemia, carbonic anhydrase inhibition.