The largest cross-sectional area and therefore lower resistance of airways?

Correct Answer: B

Rationale: Airway resistance is inversely proportional to the fourth power of the radius (Poiseuille's law), but total cross-sectional area also determines resistance across the respiratory tree. The trachea has a large diameter (~2 cm), but as a single tube, its cross-sectional area is limited (e.g., ~3-4 cm²). Bronchioles, though individually small (~1 mm), number in the thousands by the terminal stage, yet their collective area is still less than the alveoli. The alveoli, numbering ~300 million in adult lungs, have a tiny individual diameter (~0.2 mm) but an enormous total cross-sectional area (~70-100 m² during inspiration), vastly exceeding other structures. This massive area reduces airflow velocity and resistance to negligible levels at the alveolar level, where gas exchange occurs by diffusion, not flow. While resistance is highest in medium-sized bronchi due to turbulent flow, the alveoli's collective area minimizes overall resistance to air movement, making them the site of lowest resistance, contrasting with the trachea or bronchioles, which handle bulk airflow with higher resistance despite larger individual diameters.

Correct Answer: C

Rationale: Functional residual capacity (FRC) is the lung volume after a normal expiration (~2.5-3 L), the resting state where lung inward recoil balances chest wall outward recoil. It's the lung's resting volume, but also reflects the thorax's state, though these aren't mutually exclusive options. The key true statement is that at FRC, intra-alveolar pressure equals atmospheric pressure (~760 mmHg), as no airflow occurs (P = 0 gradient), and muscles are relaxed. Intrapleural pressure (IPP) at FRC is negative (~-4 mmHg, 756 mmHg), not more than atmospheric (760 mmHg), due to recoil forces keeping lungs expanded rising above atmospheric only in pathology (e.g., pneumothorax). Lung compliance varies with volume, not lowest at FRC, which is a mid-range point. The equality of alveolar and atmospheric pressure at FRC is a fundamental respiratory principle, ensuring stability at rest, making it the standout true statement.

Correct Answer: D

Rationale: When inspiratory muscles (diaphragm, external intercostals) relax, as after a normal expiration, the lungs reach functional residual capacity (FRC, ~2.5-3 L), the resting volume where lung inward recoil balances chest wall outward recoil. Vital capacity (VC, ~4-5 L) is the maximum exhailable volume after maximal inhalation, requiring active inspiration, not relaxation. Residual volume (RV, ~1-1.5 L) is the air left after maximal expiration, beyond relaxed expiration. Minimal volume' isn't a standard term but might imply RV or zero (collapsed lungs, not natural). FRC is the equilibrium state at rest, with intra-alveolar pressure equaling atmospheric (~760 mmHg), no airflow, and muscles inactive, distinguishing it as the volume post-relaxation, critical for baseline gas exchange and respiratory mechanics.

Correct Answer: B

Rationale: Surfactant, a phospholipid-protein mix from type II alveolar cells, lowers surface tension in alveoli, stabilizing them against collapse. Normally, high surface tension from water (72 dynes/cm) pulls alveoli inward, but surfactant reduces this (to ~5-10 dynes/cm), decreasing the tendency to collapse per Laplace's law (P = 2T/r). It also reduces surface tension forces directly, easing lung expansion. Lower tension decreases lymph flow by reducing fluid shifts into the interstitium from high alveolar pressures. However, lung compliance the ease of expansion increases with surfactant, as lower tension makes lungs less stiff, requiring less pressure for a given volume (C = ΔV / ΔP). Thus, compliance rises, not falls, making it the exception. This increase is vital in neonates and prevents atelectasis, contrasting with the other factors, which diminish as surfactant stabilizes alveoli and reduces mechanical stress, a key adaptation for efficient breathing.

Correct Answer: D

Rationale: Fick's law states diffusion rate = (A × D × ΔP) / d, where A is surface area, D is diffusion coefficient, ΔP is partial pressure gradient, and d is distance. For different gases (e.g., O2, CO2), the diffusion coefficient (D ∝ solubility / √MW) varies most. CO2's solubility (~0.51 ml/mmHg/L) is ~20 times O2's (~0.024 ml/mmHg/L), despite higher molecular weight (44 vs. 32), making CO2 diffuse ~20 times faster. Partial pressure gradients (e.g., O2: 100-40 mmHg, CO2: 46-40 mmHg) drive diffusion but are similar in magnitude. Temperature affects all gases uniformly in the lung (~37°C). Diffusion distance (~0.5 μm) is constant across gases. D's dominance reflects solubility's outsized role, explaining CO2's rapid equilibration vs. O2's slower rate, a critical factor in gas exchange efficiency and the most influential variable in Fick's context.