Lung Functions: Gas Exchange, pH Regulation, Defense, and More

The lungs are vital respiratory organs that perform far more than simple gas exchange. Below is a detailed overview of their major functions.

1. Gas Exchange (Primary Function)

Occurs across the alveolar-capillary membrane in ~300–500 million alveoli.

• Oxygen uptake: Inspired O₂ diffuses from alveolar air (PO₂ ~100 mmHg) into pulmonary capillary blood (PO₂ ~40 mmHg), binding to hemoglobin.
• Carbon dioxide removal: CO₂ (metabolic waste) diffuses from capillary blood (PCO₂ ~45 mmHg) into alveoli (PCO₂ ~40 mmHg) for exhalation.
• Driven by partial pressure gradients; facilitated by large surface area (~70 m²) and thin barrier (0.2–0.5 µm).

2. Regulation of Blood pH (Acid-Base Balance)

The lungs provide rapid (minutes) compensation for metabolic acid-base disturbances by adjusting CO₂ elimination.

• CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (carbonic anhydrase catalyzed).
• Hyperventilation → ↓PCO₂ → ↑pH (respiratory alkalosis).
• Hypoventilation → ↑PCO₂ → ↓pH (respiratory acidosis).
• Renal compensation (hours–days) adjusts HCO₃⁻ to normalize pH.

3. Pulmonary Surfactant Production

• Secreted by type II alveolar cells.
• Phospholipid-protein complex (dipalmitoylphosphatidylcholine + SP-A, B, C, D).
• Reduces surface tension at air-liquid interface, preventing alveolar collapse (atelectasis) at low lung volumes.
• Deficiency causes neonatal respiratory distress syndrome (NRDS); dysfunction contributes to ARDS.

4. Defense Mechanisms

• Mucociliary escalator: Goblet cells secrete mucus trapping particles; ciliated cells propel it cephalad for swallowing/expectoration.
• Alveolar macrophages: Phagocytose bacteria, viruses, inhaled particulates; release cytokines.
• Immunoglobulin A (sIgA): Neutralizes pathogens at mucosal surface.
• Cough & sneeze reflexes: Forceful expulsion of irritants.
• Antimicrobial peptides: Defensins, lysozyme, lactoferrin in airway surface liquid.

5. Additional Physiological Roles

• Thermoregulation: Heat exchange via exhaled air (minor role).
• Water balance: Insensible water loss (~300–400 mL/day) through humidified exhaled air.
• Voice production (phonation): Exhaled air passes through larynx, vibrating vocal folds; lungs provide the aerodynamic power.
• Metabolic functions: Conversion of angiotensin I to angiotensin II (via pulmonary ACE); inactivation of bradykinin, serotonin, prostaglandins; synthesis of prostaglandins, leukotrienes.
• Blood reservoir: Pulmonary circulation holds ~500 mL blood; can shift volume to systemic circulation during hemorrhage or exercise.

Clinical Relevance

Impairment of any function leads to recognizable syndromes: hypoxemia/hypercapnia (gas exchange failure), respiratory acidosis/alkalosis (pH dysregulation), atelectasis (surfactant deficiency), recurrent infections (defense failure), pulmonary hypertension (vascular remodeling). Comprehensive pulmonary assessment evaluates all these domains.