Regarding lung diseases, one of the following is true?

Correct Answer: D

Rationale: In pulmonary fibrosis, a restrictive disease, lung stiffness reduces volumes (FEV1, FVC), but the FEV1/FVC ratio remains ≥80% (normal or higher), as both drop proportionally, unlike obstructive diseases where it's <70% this is true. Airway resistance (R) ∝ 1/r^4 (Poiseuille's law); a 10% diameter increase reduces R dramatically (~40%), not increases it, making that false. COPD (e.g., emphysema, chronic bronchitis) is highly common, not least, due to smoking prevalence. Pulmonary fibrosis doesn't increase airway resistance (an obstructive feature); it reduces compliance, with resistance normal or slightly altered by volume loss. The FEV1/FVC ratio's preservation in fibrosis reflects its restrictive nature, distinguishing it as the true statement, aligning with spirometric patterns and disease mechanics.

Correct Answer: C

Rationale: Functional residual capacity (FRC) is the lung volume after a normal, quiet expiration (~2.5-3 L), where elastic recoil of the lungs (inward) balances the chest wall (outward), with no muscle activity. Residual volume (RV, ~1-1.5 L) is after maximal expiration, not quiet breathing. Expiratory reserve volume (ERV, ~1-1.5 L) is the extra air forcibly exhaled beyond normal expiration, not the resting state. Inspiratory reserve volume (IRV, ~2-3 L) is additional air inhaled beyond a normal breath, relevant to inspiration. FRC is the resting point before inspiration, maintaining alveolar patency and gas exchange efficiency, with intra-alveolar pressure equaling atmospheric (~760 mmHg). It's distinct from volumes tied to maximal efforts or active phases, reflecting the passive equilibrium critical for respiratory homeostasis.

Correct Answer: C

Rationale: After quiet expiration, respiratory muscles (diaphragm, intercostals) relax, and lungs settle at functional residual capacity (FRC, ~2.5-3 L), the resting volume where lung inward recoil equals chest wall outward recoil. Residual volume (RV, ~1-1.5 L) is post-maximal expiration, not quiet breathing. Expiratory reserve volume (ERV, ~1-1.5 L) is extra air forcibly exhaled, not the resting state. Inspiratory reserve volume (IRV, ~2-3 L) is additional inspiratory capacity, not post-expiration. FRC, with intra-alveolar pressure at atmospheric (~760 mmHg), is the passive equilibrium point, maintaining gas exchange readiness, distinct from volumes tied to forced maneuvers or inspiration, critical for respiratory baseline stability.

Correct Answer: C

Rationale: Residual volume (RV) is calculated via helium dilution, where helium doesn't enter blood, diluting based on lung volume. Spirometer volume (V1) = 12 L, initial helium concentration (C1) = 10% (0.1), final concentration (C2) = 6.67% (0.0667). Per V1C1 = V2C2: 12 × 0.1 = V2 × 0.0667, 1.2 = V2 × 0.0667, V2 = 1.2 / 0.0667 ≈ 18 L. V2 is total volume (spirometer + FRC). FRC = V2 - V1 = 18 - 12 = 6 L. FRC = ERV + RV, with ERV = 4.2 L, so RV = 6 - 4.2 = 1.8 L = 1800 ml. Equilibration at FRC (post-normal expiration) is standard. The 1800 ml reflects helium's dilution by unexpired lung air, aligning with RV's role as the non-exhalable volume, matching physiological norms.

Correct Answer: D

Rationale: When inspiratory muscles (diaphragm, external intercostals) relax post-normal expiration, lungs reach functional residual capacity (FRC, ~2.5-3 L), the resting volume where lung and chest wall recoils balance, with no airflow (alveolar pressure = 760 mmHg). Vital capacity (VC, ~4-5 L) is maximal exhalable volume, requiring active inspiration false. Residual volume (RV, ~1-1.5 L) is post-maximal expiration false. Minimal volume' isn't standard, possibly RV or collapse (not natural) false. FRC is the passive rest state, key for gas exchange baseline, distinct from volumes tied to effort, making it the correct point of muscle relaxation.