Respiratory System
Mechanics
Compliance is the change in volume per change in pressure; elastance is its reciprocal. Surfactant from type II pneumocytes lowers surface tension and prevents alveolar collapse per the law of Laplace, so premature infants develop respiratory distress with atelectasis. The diaphragm is the primary inspiratory muscle. Residual volume cannot be measured by spirometry, requiring helium dilution or body plethysmography.
Spirometry
Obstructive disease shows a reduced FEV1 with a reduced FEV1-to-FVC ratio, increased total lung capacity and residual volume, air trapping, and a flattened diaphragm. Restrictive disease reduces both FEV1 and FVC with a normal or elevated ratio and reduced total lung capacity.
Gas exchange
Ventilation-perfusion matching is the key concept: the apex has high ventilation-to-perfusion ratio (physiologic dead space) and the base has low ratio. V/Q mismatch is the most common cause of hypoxemia. Hypoxemia with a normal alveolar-arterial gradient suggests hypoventilation or low inspired oxygen; an elevated gradient suggests V/Q mismatch, shunt, or diffusion impairment. Shunt-related hypoxemia does not correct with 100 percent oxygen. Carbon monoxide poisoning causes a leftward oxygen-hemoglobin shift with normal arterial oxygen tension.
Control and disease
Central chemoreceptors respond to cerebrospinal fluid protons from carbon dioxide; peripheral carotid bodies respond primarily to hypoxemia. Asthma shows reversible obstruction with eosinophils and Curschmann spirals; COPD shows irreversible obstruction with increased compliance and air trapping. Interstitial lung disease, sarcoidosis, and pneumoconioses cause restrictive patterns with reduced diffusion capacity.
Traps
Memorizing FEV1 alone without the ratio. Assuming oxygen saturation excludes carbon monoxide poisoning. Confusing hyperventilation (low CO2) with hypoxemia.
Worked rule
Normal A-a gradient hypoxemia equals hypoventilation or low inspired oxygen; elevated A-a gradient equals V/Q mismatch, shunt, or diffusion impairment.