In normal individual, regarding gas exchange across pulmonary capillaries during mild exercise, which of the following statements is TRUE?

Correct Answer: A

Rationale: During mild exercise, pulmonary gas exchange adapts to increased O2 demand and CO2 production. CO2 diffuses ~20 times faster than O2 across the alveolar-capillary membrane due to its higher solubility (0.51 vs. 0.024 ml/mmHg/L), despite a slightly higher molecular weight (44 vs. 32), per Fick's law (D ∝ solubility / √MW) making it cross easier, a true statement. Diffusing capacity (DL) for O2 is less than for CO2 normally, and while exercise increases DL for both (recruiting capillaries), CO2's advantage persists, not O2's, and molecular weight is secondary to solubility. Capillary equilibrium length shortens for O2 and CO2 as blood flow rises, but this is nuanced and not uniquely true without context. Arterial blood gases (ABGs) remain normal in healthy individuals during mild exercise (e.g., PaO2 ~100 mmHg, PaCO2 ~40 mmHg), as ventilation matches perfusion. CO2's easier diffusion is the standout truth, rooted in its physicochemical properties, critical for efficient CO2 elimination.

Correct Answer: B

Rationale: Severe respiratory muscle weakness (e.g., in myasthenia gravis) impairs ventilation by weakening inspiratory and expiratory muscles. Tidal volume (VT, ~500 ml normally) decreases due to limited inspiratory force, reducing breath size. Vital capacity (VC, ~4-5 L) drops as maximal inhalation and exhalation are compromised. Oxyhemoglobin saturation falls (e.g., from 95-100% to <90%) as hypoventilation lowers PaO2, causing hypoxemia. Arterial pH may decrease (acidosis) if CO2 retention raises PCO2, but this isn't specified as above normal. However, PCO2 itself (normal 35-45 mmHg) rises above normal (e.g., 50-60 mmHg) due to inadequate CO2 expulsion, a direct result of weak ventilation. Though not listed, if B intended PCO2 (a common mix-up), it fits; otherwise, none are above normal' assuming intent, PCO2's rise is the key abnormality, reflecting ventilatory failure's impact on gas exchange.

Correct Answer: A

Rationale: Total lung capacity (TLC) is the maximum air lungs hold after maximal inspiration, summing residual volume (RV, ~1-1.5 L), expiratory reserve volume (ERV, ~1-1.5 L), tidal volume (VT, ~0.5 L), and inspiratory reserve volume (IRV, ~2-3 L). In adults, TLC averages ~6 L (5-7 L, varying by sex, age, size), per standard physiology (e.g., Guyton). Two liters approximates FRC (~2.5-3 L), the resting volume. Four liters nears vital capacity (VC, ~4-5 L), excluding RV. Nine liters exceeds typical capacity, possibly hyperinflation. Six liters aligns with spirometry plus RV (e.g., helium dilution), reflecting full lung expansion in health, making it the best approximation for a normal human, widely validated across respiratory studies.

Correct Answer: C

Rationale: Surfactant reduces alveolar surface tension, causing hysteresis different inflation vs. deflation pressures in lung P-V curves due to tension dynamics, a true property. It lowers pulmonary resistance by easing expansion, not increasing it false but not queried. Surfactant deficiency is common in preterm neonates (<37 weeks), causing RDS, but in term neonates (≥37 weeks), production is typically mature, making commonly deficient in term-neonates' incorrect RDS is rare at term barring defects. Surfactant indirectly prevents pulmonary edema by stabilizing alveoli, reducing fluid transudation pressure, though not its primary role true enough. The term-neonate error misaligns with developmental physiology, where surfactant sufficiency is expected, distinguishing it as the incorrect statement amid surfactant's established functions.

Correct Answer: D

Rationale: Per Poiseuille's law (R ∝ 1/r^4), a 10% airway diameter increase reduces resistance by ~40%, not increases it false. COPD (e.g., emphysema) is common due to smoking, not least false. Pulmonary fibrosis, restrictive, reduces compliance, not airway resistance (obstructive) false. In fibrosis, FEV1/FVC is ≥80% (normal or higher) as both FEV1 and FVC drop proportionally true, unlike obstructive diseases (<70%). This ratio's preservation reflects restricted volume, not airflow, a key diagnostic feature, making it the true statement amid misconceptions about resistance and prevalence.