Which of the following is the most factor that can increase the volume of air entering the lung?

Correct Answer: A

Rationale: The volume of air entering the lung during inspiration is driven by Boyle's law: lung volume increases as intrapulmonary pressure decreases relative to atmospheric pressure. This pressure gradient is created by the diaphragm and intercostal muscles contracting, expanding the thoracic cavity, and dropping intrapulmonary pressure (e.g., from 760 mmHg to 758 mmHg), allowing air to flow in. Increasing this gradient via greater muscle contraction or thoracic expansion directly increases inspired volume, making it the primary factor. Increase in action potential' is vague but likely refers to neural impulses to respiratory muscles; while more frequent or intense action potentials could enhance muscle effort, this is secondary to the mechanical pressure gradient they produce. Combining both doesn't elevate action potentials to equal status, as pressure is the direct mechanism. Decreasing the gradient reduces airflow, opposing the goal. Thus, the pressure gradient is the most critical factor, as it's the physical driver of ventilation, rooted in respiratory mechanics.

Correct Answer: D

Rationale: Pneumothorax occurs when air enters the pleural space, disrupting negative intrapleural pressure (~-4 mmHg), causing lung collapse and chest wall expansion. The thorax's diameter increases as the chest wall springs outward due to its elastic recoil. Venous return decreases because positive pleural pressure compresses the vena cava, reducing preload, especially in tension pneumothorax. Vital capacity (VC) drops as the collapsed lung reduces expirable volume (e.g., from 4-5 L to much less). However, lung compliance (C = ΔV / ΔP) doesn't increase it's a lung property (stiffness), not directly altered by pneumothorax. The collapsed lung's volume change per pressure is irrelevant, as it's deflated; compliance may appear effectively zero, but the lung tissue itself isn't more compliant. Increased compliance misrepresents pneumothorax's mechanics, where the issue is pressure loss, not lung elasticity, making this the untrue statement.

Correct Answer: B

Rationale: Alveolar ventilation (VA) is the air reaching alveoli for gas exchange: VA = (VT - VD) × RR, where VT (tidal volume) = 650 ml, VD (dead space) = 150 ml, and RR (respiratory rate) = 15 breaths/min. Calculate: VT - VD = 650 - 150 = 500 ml per breath. VA = 500 ml × 15 = 7500 ml/min = 7.5 L/min. Verify: FRC (3 L) = ERV (1.5 L) + RV (1.5 L), and TLC (8 L) = FRC + IC (VT + IRV), consistent but not needed for VA. Total ventilation (VE) = VT × RR = 650 × 15 = 9750 ml/min = 9.75 L/min, with 2.25 L/min as dead space ventilation (150 × 15), leaving 7.5 L/min as VA. This matches option B, reflecting effective gas exchange volume, critical for oxygenation and CO2 removal, aligning with standard respiratory calculations.

Correct Answer: C

Rationale: Physiological dead space (VD) is the air not participating in gas exchange, calculated via the Bohr equation: VD/VT = (PaCO2 - PECO2) / PaCO2, where VT is tidal volume (600 ml), PaCO2 is arterial PCO2 (40 mmHg), and PECO2 is mixed expired PCO2 (28 mmHg). Compute: VD/VT = (40 - 28) / 40 = 12 / 40 = 0.3. Thus, VD = 0.3 × 600 = 180 ml. Cross-check: alveolar ventilation (VA) = 4.3 L/min = (VT - VD) × RR. Assuming RR = 10/min (a reasonable resting rate), VA = 4300 ml/min ÷ 10 = 430 ml/breath, so VT - VD = 430, VD = 600 - 430 = 170 ml, close to 180 ml with rounding. The 180 ml fits directly from Bohr, reflecting both anatomical (~150 ml) and alveolar dead space, aligning with data where CO2 dilution indicates 30% of each breath is ineffective, a key metric for ventilatory efficiency.

Correct Answer: D

Rationale: Residual volume (RV) is the air remaining after maximal expiration (~1-1.5 L), preventing alveolar collapse and measurable via helium dilution or body plethysmography. Tidal volume (VT, ~500 ml) is normal breath size, not post-forceful exhalation. Expiratory reserve volume (ERV, ~1-1.5 L) is extra air exhaled beyond normal expiration, expelled during forced effort, leaving RV. Vital capacity (VC, ~4-5 L) is the maximum exhailable volume (IRV + VT + ERV), excluding RV. RV's persistence reflects lung elasticity and chest wall limits, ensuring some air stays, distinct from volumes tied to active breathing or maximal efforts, making it the correct term for this residual air critical for maintaining lung structure.