Vital capacity is defined as?

Correct Answer: D

Rationale: Vital capacity (VC) is the maximum volume of air a person can exhale after a maximal inhalation, measured via spirometry as the sum of inspiratory reserve volume (IRV, ~2-3 L), tidal volume (VT, ~0.5 L), and expiratory reserve volume (ERV, ~1-1.5 L), totaling ~4-5 L in adults. It excludes residual volume (RV), which remains after maximal exhalation. Sum of all lung volumes' describes total lung capacity (TLC, ~6 L), including RV, not VC. Tidal volume plus residual volume' (~2 L) is far less than VC, missing IRV and ERV. IRV plus ERV' omits VT, underestimating VC (~3-4 L). The correct definition IRV + VT + ERV captures the full expirable volume, reflecting the lung's functional capacity for deep breathing, a key metric in assessing respiratory health, distinguishing it from TLC or partial volume sums.

Correct Answer: A

Rationale: Airway resistance (R) follows Poiseuille's law (R ∝ 1/r^4), but total resistance depends on cross-sectional area. Later generations (bronchioles) have smaller radii, yet their vast number increases total area (e.g., ~300 cm² vs. trachea's 3-4 cm²), reducing overall R most resistance is in larger airways (trachea, bronchi), where flow is turbulent, making this false. Normally, ~80% of R is in large airways, dropping in smaller ones due to laminar flow and area. Increased R (e.g., asthma) lowers FEV1/FVC (<70%), as FEV1 falls more, a true obstructive sign. Loss of elasticity (emphysema) and bronchoconstriction (asthma) raise R by collapsing or narrowing airways, also true. The false idea of increasing R in later generations misinterprets branching dynamics, where resistance peaks proximally, not distally, aligning with physiological airflow distribution.

Correct Answer: C

Rationale: Asthma, an obstructive disease, features reversible bronchoconstriction, inflammation, and mucus production during attacks. Narrowed airways increase resistance, especially on expiration, when dynamic compression worsens airflow, producing wheezing louder and more prolonged than on inspiration, making that statement false. Work of breathing rises as respiratory muscles (e.g., diaphragm) work harder against resistance and trapped air, a consistent expectation. Bronchodilators (e.g., albuterol) are standard treatment, relaxing bronchial smooth muscle, not contraindicated. FEV1 decreases (e.g., from 80% to 50% predicted) due to obstructed airflow, not increases. Increased work of breathing reflects the effort to overcome narrowed passages, elevating energy expenditure and often leading to accessory muscle use, aligning with asthma's acute physiology where resistance and air trapping dominate, distinguishing it from incorrect options misaligned with clinical presentation.

Correct Answer: A

Rationale: Air enters the lungs during inspiration per Boyle's law: thoracic expansion lowers intrapulmonary pressure below atmospheric (e.g., 760 to 758 mmHg), creating a pressure gradient driving airflow. Increasing this gradient via stronger diaphragm and intercostal contraction directly boosts inspired volume (e.g., from 500 ml to 600 ml), the primary factor. Increase in action potential' likely means neural impulses to respiratory muscles; more impulses enhance contraction, but this is secondary to the gradient they produce. Combining both overcomplicates pressure is the direct mechanism. Decreasing the gradient reduces flow, opposing the goal. The pressure gradient is the key driver, quantifiable (e.g., 1-2 mmHg for tidal breathing), linking muscle action to volume via physics, distinguishing it as the most impactful factor in ventilation mechanics.

Correct Answer: D

Rationale: Minute ventilation (VE) = tidal volume (VT) × respiratory rate (RR), fixed at 6 L/min (6000 ml/min). Alveolar ventilation (VA) = (VT - VD) × RR, with anatomic dead space (VD) = 150 ml. For 200 ml at 30/min: VE = 200 × 30 = 6000 ml/min, VA = (200 - 150) × 30 = 50 × 30 = 1500 ml/min. For 300 ml at 20/min: VA = (300 - 150) × 20 = 150 × 20 = 3000 ml/min. For 400 ml at 15/min: VA = (400 - 150) × 15 = 250 × 15 = 3750 ml/min. For 600 ml at 10/min: VA = (600 - 150) × 10 = 450 × 10 = 4500 ml/min. The 600 ml at 10/min maximizes VA (4.5 L/min), as larger VT exceeds VD more, despite lower RR. High RR with low VT wastes ventilation in dead space, reducing VA efficiency. This deeper, slower pattern optimizes gas exchange, making it the best choice.