Which of the following structures contains blood with the highest PCO2?

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: 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.

Correct Answer: D

Rationale: When inspiratory muscles (diaphragm, external intercostals) relax post-normal expiration, lungs reach functional residual capacity (FRC, ~2.5-3 L), the resting volume where lung and chest wall recoils balance, with no airflow (alveolar pressure = 760 mmHg). Vital capacity (VC, ~4-5 L) is maximal exhalable volume, requiring active inspiration false. Residual volume (RV, ~1-1.5 L) is post-maximal expiration false. Minimal volume' isn't standard, possibly RV or collapse (not natural) false. FRC is the passive rest state, key for gas exchange baseline, distinct from volumes tied to effort, making it the correct point of muscle relaxation.

Correct Answer: C

Rationale: Convection is the primary heat transfer mechanism from Earth's surface to the atmosphere. Solar radiation heats the surface (~168 W/m² absorbed), warming air via conduction (~24 W/m²), but convection dominates as warm air rises, transferring heat vertically (~97 W/m² latent + sensible heat, per energy budgets like Trenberth). Radiation (~396 W/m² emitted, ~333 W/m² back via greenhouse) is surface-to-space, not directly to atmosphere. Conduction is minor due to air's low conductivity. Advection moves heat horizontally, not vertically primary. Convection's buoyancy-driven circulation (e.g., thunderstorms) outpaces other modes, making it the key process, per climate models, for atmospheric heating.