Which of the following will the have the highest percentage of CO2?

Correct Answer: B

Rationale: The percentage of CO2 in a structure reflects its PCO2, tied to metabolic production and gas exchange. Pulmonary arteries carry deoxygenated blood from the right heart to the lungs, with a PCO2 of ~45-46 mmHg (venous blood), the highest among options, as it's loaded with CO2 from systemic tissues. Alveolar air has a PCO2 of ~40 mmHg, equilibrated with arterial blood after CO2 diffuses out during respiration. Pulmonary veins, post-gas exchange, carry oxygenated blood with a PCO2 of ~40 mmHg, matching arterial levels. Interstitial fluid's PCO2 varies but approximates venous blood (~45 mmHg) or slightly less, depending on local metabolism, though it's not a standard respiratory measure. Systemic arteries, not listed, also have ~40 mmHg. Pulmonary arteries stand out with the highest CO2 due to their role in transporting metabolically produced CO2 to the lungs for excretion, before alveolar ventilation lowers it, making them the site of peak CO2 concentration.

Correct Answer: D

Rationale: When inspiratory muscles (diaphragm, external intercostals) relax, as after a normal expiration, the lungs reach functional residual capacity (FRC, ~2.5-3 L), the resting volume where lung inward recoil balances chest wall outward recoil. Vital capacity (VC, ~4-5 L) is the maximum exhailable volume after maximal inhalation, requiring active inspiration, not relaxation. Residual volume (RV, ~1-1.5 L) is the air left after maximal expiration, beyond relaxed expiration. Minimal volume' isn't a standard term but might imply RV or zero (collapsed lungs, not natural). FRC is the equilibrium state at rest, with intra-alveolar pressure equaling atmospheric (~760 mmHg), no airflow, and muscles inactive, distinguishing it as the volume post-relaxation, critical for baseline gas exchange and respiratory mechanics.

Correct Answer: B

Rationale: Surfactant, a phospholipid-protein mix from type II alveolar cells, lowers surface tension in alveoli, stabilizing them against collapse. Normally, high surface tension from water (72 dynes/cm) pulls alveoli inward, but surfactant reduces this (to ~5-10 dynes/cm), decreasing the tendency to collapse per Laplace's law (P = 2T/r). It also reduces surface tension forces directly, easing lung expansion. Lower tension decreases lymph flow by reducing fluid shifts into the interstitium from high alveolar pressures. However, lung compliance the ease of expansion increases with surfactant, as lower tension makes lungs less stiff, requiring less pressure for a given volume (C = ΔV / ΔP). Thus, compliance rises, not falls, making it the exception. This increase is vital in neonates and prevents atelectasis, contrasting with the other factors, which diminish as surfactant stabilizes alveoli and reduces mechanical stress, a key adaptation for efficient breathing.

Correct Answer: D

Rationale: Fick's law states diffusion rate = (A × D × ΔP) / d, where A is surface area, D is diffusion coefficient, ΔP is partial pressure gradient, and d is distance. For different gases (e.g., O2, CO2), the diffusion coefficient (D ∝ solubility / √MW) varies most. CO2's solubility (~0.51 ml/mmHg/L) is ~20 times O2's (~0.024 ml/mmHg/L), despite higher molecular weight (44 vs. 32), making CO2 diffuse ~20 times faster. Partial pressure gradients (e.g., O2: 100-40 mmHg, CO2: 46-40 mmHg) drive diffusion but are similar in magnitude. Temperature affects all gases uniformly in the lung (~37°C). Diffusion distance (~0.5 μm) is constant across gases. D's dominance reflects solubility's outsized role, explaining CO2's rapid equilibration vs. O2's slower rate, a critical factor in gas exchange efficiency and the most influential variable in Fick's context.

Correct Answer: B

Rationale: Mixed expired PCO2 (PECO2) reflects exhaled CO2 diluted by dead space. If dead space (VD) is one-third tidal volume (VT), VD/VT = 1/3. Per Bohr's equation: VD/VT = (PaCO2 - PECO2) / PaCO2, with PaCO2 = 45 mmHg. Then: 1/3 = (45 - PECO2) / 45, so 45 / 3 = 45 - PECO2, 15 = 45 - PECO2, PECO2 = 30 mmHg. Assuming physiological dead space equals anatomic here (no alveolar dead space specified), one-third of each breath (~0 mmHg CO2 from inspired air) dilutes the alveolar CO2 (~45 mmHg) to two-thirds strength (30 mmHg). A 45 mmHg PECO2 implies no dead space, while 20 mmHg over-dilutes. The 30 mmHg fits the ratio and respiratory mechanics, showing how dead space lowers expired CO2 relative to arterial levels, a key ventilatory efficiency measure.