Respiratory Mechanics and Gas Exchange covers respiratory mechanics and gas exchange for INI CET (AIIMS PG).
Respiratory Muscles:
- Inspiration: Diaphragm (main — accounts for ~75% of TV during quiet breathing), external intercostals (accessory)
- Expiration: Normally passive (elastic recoil of lungs and chest wall); forced expiration → internal intercostals + abdominal muscles
Thoracic Cavity Pressures:
- Intrapleural pressure (Ppl): Always negative (subatmospheric) — keeps lungs inflated; becomes MORE negative during inspiration
- At rest (FRC): –5 cmH₂O; During inspiration: –8 cmH₂O
- Pneumothorax: Ppl = atmospheric → lung collapses
- Intra-alveolar pressure (Palv): Must become negative for air to enter lungs
- During inspiration: –1 cmH₂O (slight negativity draws air in); During expiration: +1 cmH₂O
- Transpulmonary pressure: Palv – Ppl = lung distending pressure
Lung Volumes and Capacities (on spirometry):
| Volume/Capacity | Value (male) | Description |
|---|---|---|
| TV (Tidal Volume) | ~500 mL | Normal breath |
| IRV (Inspiratory Reserve) | ~3000 mL | Max inspiration from TV |
| ERV (Expiratory Reserve) | ~1200 mL | Max expiration from FRC |
| RV (Residual Volume) | ~1200 mL | Cannot be exhaled |
| IC = TV + IRV | ~3500 mL | Max inspiration from FRC |
| FRC = ERV + RV | ~2400 mL | Lung volume at rest |
| VC = TV + IRV + ERV | ~4800 mL | Max voluntary ventilation |
| TLC = all four | ~6000 mL | Max lung inflation |
Compliance: Change in volume per unit change in pressure
- Static compliance: ΔV / ΔP at no flow (normal: ~200 mL/cmH₂O)
- Dynamic compliance: Measured during airflow
- Decreased compliance (stiff lungs): Pulmonary fibrosis, ARDS, neonatal respiratory distress syndrome (surfactant deficiency)
- Increased compliance (floppy lungs): Emphysema (loss of elastic tissue)
Surface Tension and Surfactant:
- Surface tension: Alveoli lined with liquid; water molecules attract each other → tendency to collapse alveoli (LaPlace’s Law: P = 2T/r — small alveoli have higher pressure)
- Surfactant: Produced by Type II pneumocytes; mixture of dipalmitoylphosphatidylcholine (DPPC) + proteins (SP-A, SP-B, SP-C, SP-D)
- Function of surfactant: Reduces surface tension; more effective in small alveoli (adjusts with radius) → prevents atelectasis; increases compliance
- Neonatal Respiratory Distress Syndrome (NRDS): Deficiency of surfactant → ↑surface tension → alveolar collapse → hyaline membrane disease; common in premature infants (due at <28 weeks — insufficient surfactant production); treated with exogenous surfactant + maternal corticosteroids
Gas Exchange (Fick’s law of diffusion):
- Alveolar gas equation: PAO₂ = FiO₂(P_ATM – PH₂O) – (PaCO₂ / R)
- Diffusion: All gases equilibrate across alveolar-capillary membrane in ~0.25 seconds; RBC transit time is ~0.75 seconds — so normal gas exchange is complete well before RBC leaves capillary
- V/Q mismatch: Primary cause of hypoxemia (low PAO₂)
- V/Q = 0 ( shunt): Blood passes through lung without gas exchange (e.g., congenital shunt, atelectasis) — does NOT respond to O₂
- V/Q = ∞ (dead space): Alveoli ventilated but not perfused (e.g., PE) — does respond to O₂
⚡ Exam Tip for INI CET (AIIMS PG): COPD (emphysema) = ↓elastic recoil + airway obstruction → ↓FEV1/FVC ratio (<0.7); air trapping → ↑RV and ↑TLC. Restrictive lung disease (pulmonary fibrosis) = ↓compliance → ↓FVC and ↓FEV1 (proportionally reduced) → normal or ↑FEV1/FVC ratio.