At the end of normal quiet expiration, just before the start of inspiration, the lungs are said to be in:

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: 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.

Correct Answer: A

Rationale: The pterygopalatine fossa is a key junction. The maxillary artery enters via the pterygomaxillary fissure (A) from the infratemporal fossa, branching to supply the nose, palate, and pharynx. The maxillary nerve (V2) enters from the middle cranial fossa (C) through the foramen rotundum, exiting variably (e.g., infraorbital canal, D, for the face). At the pterygomaxillary fissure (A), the artery enters while the nerve is already within, heading elsewhere, fulfilling different directions.' B (infratemporal fossa) is the artery's origin, not passage point; C and D are partial paths, not the junction. A is the precise anatomical site of divergence.