Regarding pulmonary vascular resistance, which one of the following is true?

Correct Answer: B

Rationale: Pulmonary vascular resistance (PVR) is the resistance to blood flow through the pulmonary circulation, influenced by lung volume and vessel mechanics. At high lung volumes (e.g., near total lung capacity), extra-alveolar vessels are stretched and narrowed due to lung expansion, increasing PVR, while alveolar capillaries are compressed, further elevating resistance. Conversely, at low lung volumes (e.g., near residual volume), extra-alveolar vessels are less stretched and more open, and alveolar capillaries are less compressed, resulting in lower PVR. Thus, PVR is lowest at low lung volumes, making that statement true. Increased PVR, as seen in conditions like pulmonary hypertension or fibrosis, can indeed strain the right heart, leading to failure (cor pulmonale), but this is not the focus of the true statement query. PVR is not measured by routine pulmonary function tests (e.g., spirometry), which assess airflow and volumes, not vascular pressures; it requires invasive methods like cardiac catheterization. The statement about low PVR at low lung volumes reflects the physiological relationship between lung volume and vascular caliber, where resistance is minimized when the lungs are less expanded.

Correct Answer: C

Rationale: At the end of quiet expiration, respiratory muscles (diaphragm, intercostals) relax, and the lungs reach functional residual capacity (FRC), typically 2.5-3 liters. FRC is the resting volume where lung inward recoil balances chest wall outward recoil, maintaining equilibrium without active effort. Residual volume (RV, ~1-1.5 L) is the air left after maximal expiration, not quiet breathing. Expiratory reserve volume (ERV, ~1-1.5 L) is the extra air forcibly exhaled beyond quiet expiration, not present at rest. Inspiratory reserve volume (IRV, ~2-3 L) is additional air inhaled beyond a normal breath, irrelevant post-expiration. Total lung capacity (TLC, ~6 L) includes all volumes, not the resting state. FRC's role as the baseline volume after passive expiration reflects the natural relaxation point, critical for continuous gas exchange, distinguishing it from volumes tied to forced maneuvers or inspiration.

Correct Answer: C

Rationale: Residual volume (RV) is the air left after maximal expiration, not measurable by spirometry but calculable via helium dilution. Here, a 12 L spirometer with 10% helium (C1 = 0.1) equilibrates with lung volume (initially FRC). Post-equilibration, expired gas is 6.67% helium (C2 = 0.0667). Using V1C1 = V2C2 (helium conservation), V1 = 12 L, C1 = 0.1, C2 = 0.0667: 12 × 0.1 = V2 × 0.0667, so 1.2 = V2 × 0.0667, V2 = 1.2 / 0.0667 ≈ 18 L. V2 is total gas volume (spirometer + FRC). FRC = V2 - V1 = 18 - 12 = 6 L. FRC = ERV + RV, and ERV = 4.2 L, so RV = FRC - ERV = 6 - 4.2 = 1.8 L = 1800 ml. This assumes equilibration at FRC (post-normal expiration), common in such problems. The 1800 ml matches helium dilution principles, where dilution reflects unexpired lung volume, confirming RV amidst the options.

Correct Answer: D

Rationale: Normal inhalation follows a mechanical sequence. (5) Contraction of the diaphragm and external intercostals starts it, expanding the thoracic cavity. (1) This lowers intrapleural pressure (IPP) from -4 mmHg (756 mmHg) to -6 mmHg (754 mmHg), increasing transpulmonary pressure. (3) Lung size increases as the lungs expand with the chest wall. (4) Intra-alveolar pressure drops to 759 mmHg (-1 mmHg) as volume rises (Boyle's law), creating a gradient from atmospheric pressure (760 mmHg). (2) Air flows in from higher to lower pressure. The order 5,1,3,4,2 reflects causality: muscle action lowers IPP, expands lungs, drops alveolar pressure, and drives airflow. Alternatives disrupt this: 5,2,3,4,1 puts flow before pressure changes; 1,3,4,5,2 starts with IPP drop without muscle action; 5,4,3,2,1 misplaces alveolar pressure before lung expansion. The correct sequence mirrors respiratory physiology's step-by-step process.

Correct Answer: B

Rationale: Idiopathic pulmonary fibrosis (IPF) scars the lung interstitium, reducing elasticity and volumes. Total lung capacity (TLC) decreases (e.g., from 6 L to 4 L) as stiff lungs resist expansion. FEV1 and FVC both drop due to restricted capacity, though their ratio (FEV1/FVC) stays normal or high (≥80%). Arterial PO2 (PaO2) falls (e.g., from 75-100 mmHg to 60 mmHg) due to impaired diffusion across thickened alveoli, causing hypoxemia. However, total pulmonary vascular resistance (PVR) increases as fibrosis compresses and obliterates capillaries, narrowing the vascular bed and raising resistance to blood flow. This can strain the right heart, potentially leading to cor pulmonale, a known IPF complication. Among these, only PVR exceeds normal levels, reflecting the disease's vascular impact, while volumes and oxygenation decline, aligning with IPF's restrictive pattern and distinguishing it from healthy physiology.