The work of breathing is:

Correct Answer: A

Rationale: Work of breathing (WOB) is the energy required to overcome elastic (compliance) and resistive (airway) forces during ventilation. Lung compliance (C = ΔV / ΔP) measures lung stretchability; low compliance (stiff lungs) increases pressure needed for a given volume, raising WOB. Thus, WOB is inversely related to compliance when C decreases, WOB increases, as in fibrosis. During exercise, WOB rises with higher ventilation rates and volumes, not remaining constant. Airway resistance (R) directly affects WOB; higher R (e.g., asthma) increases effort, contradicting not affected.' In pulmonary fibrosis, stiff lungs (low compliance) elevate WOB, not reduce it, unlike emphysema where high compliance might lower elastic work but raise resistive work. The inverse compliance relationship is fundamental, as WOB = ∫P dV, where pressure (P) rises as compliance falls, making this the correct statement reflecting respiratory mechanics.

Correct Answer: C

Rationale: A stab wound causing pneumothorax allows air into the pleural space, disrupting the negative intrapleural pressure (~-4 to -6 mmHg) that keeps lungs expanded. This equalizes pleural pressure to atmospheric (760 mmHg), eliminating the force opposing lung elastic recoil, which pulls the lung inward to collapse toward the hilum, reducing its volume. Meanwhile, the chest wall's outward recoil, no longer countered by lung tension, causes it to expand outward, increasing thoracic diameter. Thus, the lung collapses and the chest wall expands, a classic pneumothorax feature. Both expanding defies recoil mechanics, fixing at FRC ignores pressure loss, and collapse-collapse misrepresents chest wall behavior. This dynamic reflects the opposing elastic properties unleashed by pleural breach, critical for understanding respiratory compromise and interventions like chest tube placement.

Correct Answer: D

Rationale: Inspiration relies on Boyle's law: expanding the thorax lowers intrapulmonary pressure (e.g., 760 to 758 mmHg) below atmospheric, creating a pressure difference driving air in. Diaphragm and intercostal contraction generate this ~1-2 mmHg gradient for tidal breathing (~500 ml). Atmospheric pressure (760 mmHg) is static, not a force its difference with intrapulmonary pressure matters. Muscular spasm implies involuntary action, unlike controlled respiratory muscle contraction. Reduced surface tension (via surfactant) eases expansion but isn't the force pressure difference is. Muscle relaxation drives expiration, not inspiration. This gradient, directly linking mechanics to airflow, is the primary force, quantifiable and fundamental to ventilation, distinguishing it from secondary factors like surfactant or muscle state.

Correct Answer: D

Rationale: Restrictive lung disease (e.g., fibrosis) stiffens lungs, reducing expansion. Forced vital capacity (FVC) drops (e.g., from 4-5 L to 2-3 L) due to limited volume. FEV1 also falls (e.g., 3-4 L to 1.5-2 L) proportionally, but the FEV1/FVC ratio stays normal or high (≥80%), as both decrease similarly, unlike obstructive disease (<70%). Ventilation/perfusion (V/Q) ratio may rise in fibrosis (ventilation falls more than perfusion), not remaining normal. FEV1 and FVC individually are reduced, not normal. The FEV1/FVC ratio's preservation is a restrictive hallmark volume-limited, not airflow-obstructed making it the relatively normal value, key for spirometric diagnosis and distinguishing restrictive from obstructive patterns.

Correct Answer: A

Rationale: During mild exercise, gas exchange meets increased O2 demand and CO2 output. CO2 diffuses ~20 times faster than O2 across the alveolar-capillary membrane due to higher solubility (0.51 vs. 0.024 ml/mmHg/L), despite molecular weight (44 vs. 32), per Fick's law (D ∝ solubility / √MW) true. Diffusing capacity (DL) for O2 is less than CO2's; exercise boosts both via capillary recruitment, but solubility, not weight, drives CO2's edge false. Capillary equilibrium length may shorten with faster flow, but this is nuanced, not definitive false. Arterial blood gases (ABGs) stay normal (PaO2 ~100 mmHg, PaCO2 ~40 mmHg) in health during mild exercise false. CO2's easier diffusion, rooted in solubility, ensures rapid CO2 clearance, a key truth distinguishing gas exchange dynamics in exercise physiology.