Regarding physiological dead space, one of the following is wrong?

Correct Answer: D

Rationale: Physiological dead space (VDphys) includes anatomic dead space (VDanat, ~150 ml, conducting airways) plus alveolar dead space (VDalv, ventilated but non-perfused alveoli). Normally, VDphys ≈ VDanat (~150 ml), but in disease, it's equal to or greater due to added VDalv. Lung diseases like pulmonary embolism increase VDphys by raising VDalv from poor perfusion. A high V/Q ratio (ventilation > perfusion), as in PE, also increases VDphys, as ventilated alveoli lack blood flow. However, VDphys isn't equal to alveolar dead space alone VDalv is just one component. VDphys = VDanat + VDalv, so stating it equals VDalv excludes the anatomic portion, which is always present (e.g., trachea, bronchi). This misdefinition is wrong, as physiological dead space encompasses both, not just wasted alveolar volume, a distinction critical for understanding gas exchange inefficiencies in pathology.

Correct Answer: C

Rationale: PCO2, the partial pressure of carbon dioxide, indicates CO2 concentration in blood, highest where metabolic waste accumulates before gas exchange. The superior vena cava (SVC) carries deoxygenated blood from the upper body to the right atrium, with a PCO2 of ~45-46 mmHg venous blood rich in CO2 from tissue metabolism, making it the highest here. Pulmonary veins carry oxygenated blood post-alveolar exchange, with PCO2 lowered to arterial levels (~40 mmHg). Midportion pulmonary capillaries are transitional, where PCO2 drops from venous (~46 mmHg) to arterial (~40 mmHg) during gas exchange, averaging less than SVC. Carotid bodies, chemoreceptors sensing arterial blood (PCO2 ~40 mmHg), aren't blood reservoirs. SVC's role in collecting systemic venous return ensures it carries the most CO2-rich blood before pulmonary offloading, distinguishing it from oxygenated or exchanging sites, reflecting the circulatory path where CO2 peaks prior to exhalation.

Correct Answer: C

Rationale: Functional residual capacity (FRC) is the lung volume after a normal, quiet expiration (~2.5-3 L), where elastic recoil of the lungs (inward) balances the chest wall (outward), with no muscle activity. Residual volume (RV, ~1-1.5 L) is after maximal expiration, not quiet breathing. Expiratory reserve volume (ERV, ~1-1.5 L) is the extra air forcibly exhaled beyond normal expiration, not the resting state. Inspiratory reserve volume (IRV, ~2-3 L) is additional air inhaled beyond a normal breath, relevant to inspiration. FRC is the resting point before inspiration, maintaining alveolar patency and gas exchange efficiency, with intra-alveolar pressure equaling atmospheric (~760 mmHg). It's distinct from volumes tied to maximal efforts or active phases, reflecting the passive equilibrium critical for respiratory homeostasis.

Correct Answer: C

Rationale: After quiet expiration, respiratory muscles (diaphragm, intercostals) relax, and lungs settle at functional residual capacity (FRC, ~2.5-3 L), the resting volume where lung inward recoil equals chest wall outward recoil. Residual volume (RV, ~1-1.5 L) is post-maximal expiration, not quiet breathing. Expiratory reserve volume (ERV, ~1-1.5 L) is extra air forcibly exhaled, not the resting state. Inspiratory reserve volume (IRV, ~2-3 L) is additional inspiratory capacity, not post-expiration. FRC, with intra-alveolar pressure at atmospheric (~760 mmHg), is the passive equilibrium point, maintaining gas exchange readiness, distinct from volumes tied to forced maneuvers or inspiration, critical for respiratory baseline stability.

Correct Answer: C

Rationale: Residual volume (RV) is calculated via helium dilution, where helium doesn't enter blood, diluting based on lung volume. Spirometer volume (V1) = 12 L, initial helium concentration (C1) = 10% (0.1), final concentration (C2) = 6.67% (0.0667). Per V1C1 = V2C2: 12 × 0.1 = V2 × 0.0667, 1.2 = V2 × 0.0667, V2 = 1.2 / 0.0667 ≈ 18 L. V2 is total volume (spirometer + FRC). FRC = V2 - V1 = 18 - 12 = 6 L. FRC = ERV + RV, with ERV = 4.2 L, so RV = 6 - 4.2 = 1.8 L = 1800 ml. Equilibration at FRC (post-normal expiration) is standard. The 1800 ml reflects helium's dilution by unexpired lung air, aligning with RV's role as the non-exhalable volume, matching physiological norms.