Respiratory Physiology
The respiratory system maintains gas exchange through ventilation, diffusion, and perfusion, tightly regulated to maintain PaO₂ 80-100 mmHg and PaCO₂ 35-45 mmHg. Ventilation: Tidal volume (500 mL) × respiratory rate...
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- Respiratory failure with hypoxaemia and hypercapnia
- Acute respiratory distress syndrome (ARDS)
- Severe bronchospasm and air trapping
- Pneumothorax causing respiratory compromise
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- ANZCA Primary Written
- ANZCA Primary Viva
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Respiratory physiology provides the foundation for understanding mechanical ventilation, oxygen therapy, and pulmonary p... CICM Fellowship Written, CICM Fellow
The respiratory system maintains gas exchange through ventilation, diffusion, and perfusion, tightly regulated to maintain PaO₂ 80-100 mmHg and PaCO₂ 35-45 mmHg. Ventilation: Tidal volume (500 mL) × respiratory rate...
Quick Answer
The respiratory system maintains gas exchange through ventilation, diffusion, and perfusion, tightly regulated to maintain PaO₂ 80-100 mmHg and PaCO₂ 35-45 mmHg. Ventilation: Tidal volume (500 mL) × respiratory rate (12-16/min) = minute ventilation (6-8 L/min); alveolar ventilation (tidal volume − dead space 150 mL) × rate = 4-5 L/min; dead space (conducting airways, no gas exchange) approximately 2 mL/kg. Lung volumes: TLC 6-7 L (total lung capacity), VC 4-5 L (vital capacity), FRC 2.5-3.5 L (functional residual capacity—volume at end-expiration, oxygen reserve), RV 1.5-2 L (residual volume); closing capacity (volume at which small airways close) increases with age, obesity, supine position. Compliance: Distensibility of lung (ΔV/ΔP); normal 100 mL/cm H₂O; reduced in ARDS, pulmonary oedema, fibrosis; increased in emphysema. Diffusion: O₂ and CO₂ exchange across alveolar-capillary membrane; diffusion capacity (DLCO) measures efficiency; impaired by thickening (fibrosis, oedema), reduced surface area (emphysema), reduced capillary blood volume. Perfusion: Pulmonary blood flow 5 L/min (all cardiac output); West zones (I—alveolar pressure > arterial, no flow; II—arterial > alveolar > venous, intermittent flow; III—arterial > venous > alveolar, continuous flow); V/Q matching essential for efficient gas exchange (normal V/Q 0.8). Hypoxic pulmonary vasoconstriction (HPV): Precise mechanism diverts blood away from poorly ventilated alveoli to well-ventilated areas; inhibited by vasodilators (volatile agents, nitrates), one-lung ventilation relies on HPV to reduce shunt. Oxygen transport: Dissolved (2% at 100 mmHg PaO₂), bound to haemoglobin (98%, 1 g Hb binds 1.34 mL O₂, SaO₂ depends on PaO₂ via oxyhaemoglobin dissociation curve—right shift by CO₂, acid, temperature, 2,3-DPG favours release to tissues). CO₂ transport: Dissolved (5%), carbamino compounds (5%), bicarbonate (90%); Henderson-Hasselbalch equation relates CO₂, HCO₃⁻, pH; hypercapnia stimulates central and peripheral chemoreceptors increasing ventilation. Anaesthetic implications: All anaesthetics depress ventilation (reduced response to CO₂ and hypoxia); FRC reduced by 20% with induction (loss of respiratory muscle tone, diaphragm cephalad displacement); atelectasis develops quickly—recruitment manoevers (PEEP 40 cm H₂O for 40 seconds) reduce this; intubation bypasses dead space; mechanical ventilation must match metabolic needs. Obesity/pregnancy: FRC reduced, desaturation rapid; head-up position improves FRC. Indigenous populations: Higher smoking rates, COPD prevalence; careful assessment essential. [1-10]