Which of the following regarding Residual volume is correct?

Correct Answer: D

Rationale: Residual volume (RV) is the air remaining in the lungs after a maximal expiration, preventing complete collapse and maintaining alveolar stability. It's not just after tidal expiration (that's FRC), so that statement is imprecise. In COPD, an obstructive disease, air trapping increases RV due to impaired expiration from narrowed airways or loss of elastic recoil, not decreases it. In pulmonary fibrosis, a restrictive disease, RV decreases because stiff lungs limit all volumes, including the amount left after maximal effort. However, RV remains relatively constant throughout a healthy person's life, unaffected by aging alone in the absence of disease, as lung elasticity and structure don't drastically alter RV without pathology. While lung volumes like vital capacity may shift with age, RV's stability reflects its role as a fixed baseline, measured indirectly (e.g., helium dilution), and isn't subject to significant physiological variation over time in health. Thus, its consistency across a lifetime is the correct statement, distinguishing it from disease-specific changes.

Correct Answer: C

Rationale: Surfactant, from type II alveolar cells, reduces alveolar surface tension, aiding lung function. It causes hysteresis the difference in lung inflation vs. deflation pressure-volume curves by lowering tension more effectively as alveoli expand, a true property. It decreases, not increases, pulmonary resistance by easing expansion and reducing collapse tendency, so that's incorrect but not the queried option. Surfactant deficiency is common in preterm neonates (<37 weeks), causing respiratory distress syndrome (RDS), but in term neonates (≥37 weeks), surfactant production is typically mature, making commonly deficient in term-neonates' incorrect RDS is rare at term unless congenital defects exist. Surfactant prevents pulmonary edema indirectly by maintaining alveolar stability, reducing transudation pressure, though not its primary role. The term-neonate statement is the incorrect one, misaligning with developmental physiology where surfactant sufficiency is expected at full gestation.

Correct Answer: D

Rationale: Pulmonary function tests (PFTs) differentiate lung diseases. Obstructive diseases (e.g., COPD) reduce airflow, decreasing FEV1 (<80% predicted) due to airway narrowing, with increased residual volume (RV) and total lung capacity (TLC) from air trapping. Restrictive diseases (e.g., fibrosis) limit expansion, also reducing FEV1 (<80% predicted) due to lower volumes, but RV and TLC decrease. Decreased FEV1 is common to both obstructive from airflow limitation, restrictive from reduced capacity making it consistent across types. Decreased RV fits restriction, not obstruction (increased RV). Normal or above TLC fits obstruction, not restriction (decreased TLC). Vascular resistance isn't a PFT metric; it rises in fibrosis, not decreases. Decreased FEV1's shared reduction reflects impaired exhalation, a unifying feature despite differing mechanisms, distinguishing it as the overlapping value.

Correct Answer: B

Rationale: Infant respiratory distress syndrome (IRDS) in premature babies stems from inadequate surfactant production, critical for reducing alveolar surface tension (T). Without surfactant, T increases, causing alveoli to collapse due to high water-induced tension, unlike normal low-tension stability. This elevates the pressure needed to expand lungs, decreasing lung compliance (C), as stiff lungs resist inflation a hallmark of IRDS. Collapsed alveoli impair gas exchange, reducing arterial PO2 (PaO2) from normal (75-100 mmHg) to hypoxic levels (e.g., 50-60 mmHg), driving respiratory distress. Option B (T increases, C decreases, PaO2 decreases) matches this pathophysiology: high T from surfactant lack, low C from rigidity, and low PaO2 from poor oxygenation. Other options fail e.g., C increasing contradicts stiffness, PaO2 equal ignores hypoxemia. This triad reflects IRDS's core mechanism, where surfactant deficiency cascades into ventilatory and oxygenation failure.

Correct Answer: B

Rationale: Atmospheric air has ~21% oxygen (O2) and ~0.04% carbon dioxide (CO2), so O2's percentage vastly exceeds CO2's, reflecting their natural abundances. Solubility, per Henry's law, depends on the solubility coefficient: O2's is ~0.024 ml/mmHg/L blood, while CO2's is ~0.51 ml/mmHg/L over 20 times higher. Thus, O2 is less soluble than CO2, despite its higher atmospheric presence. In blood, O2 relies on hemoglobin (~98% bound, ~2% dissolved), while CO2 uses dissolved (~7%), bicarbonate (~70%), and hemoglobin (~23%) forms, leveraging its solubility. Option B (higher O2 percentage, lower O2 solubility) fits: 21% vs. 0.04%, and 0.024 vs. 0.51. This contrast drives distinct transport mechanisms O2's hemoglobin dependence vs. CO2's solubility advantage crucial for respiration and acid-base balance, making it the accurate physiological description.