The diffusion constant is proportional to:

Correct Answer: D

Rationale: the diffusion constant (D) is proportional to gas solubility (Fick's law: V = D·A·ΔP/T, where D ∝ solubility/MW). Choice A is false; D is inversely proportional to thickness (T). Choice B is wrong; D is inversely proportional to √MW (Graham's law component). ' D is independent of partial pressure difference (ΔP drives flux, not D). Solubility, critical for O₂ (low) vs. CO₂ (high), determines D's magnitude (e.g., DₗCO₂ ≈ 20x DₗO₂), making E the accurate factor in lung diffusion.

Correct Answer: A

Rationale: central chemoreceptors respond to blood CO₂ via CSF pH changes (CO₂ diffuses, forms H⁺), not directly to O₂ or H⁺. Choice B (carotid) and C (aortic) sense PO₂, PCO₂, and pH. Choice D (stretch) is mechanical. Located in the medulla, central receptors drive 70-80% of CO₂ response, insensitive to hypoxia, making A the specific match.

Correct Answer: D

Rationale: CO is diffusion-limited its high Hb affinity and low solubility prevent equilibration in 0.75 s transit. Choice A (Oâ‚‚) is perfusion-limited at rest (equilibrates in 0.25 s). Choice B (Nâ‚‚O) is perfusion-limited (high solubility). Choice C (COâ‚‚) equilibrates fast (20x Oâ‚‚ diffusion). Choice E (halothane) is perfusion-limited. CO's slow uptake (Dâ‚—CO measures lung capacity) distinguishes D as diffusion-limited.

Correct Answer: D

Rationale: advancing age increases compliance (loss of elastic recoil, e.g., emphysema-like), not decreases. Choice A (LV failure) reduces it via edema (stiffer lungs). Choice B (atelectasis) decreases via collapse. Choice C (fibrosis) lowers it (scarring). Choice E (raised PCWP) mimics edema. Aging weakens lung elastin, raising compliance (e.g., 250 vs. 200 mL/cm Hâ‚‚O), distinguishing D as the exception.

Correct Answer: C

Rationale: nitrous oxide (N₂O) is perfusion-limited, not diffusion-limited its high solubility allows rapid equilibration (≈0.1 s) with pulmonary blood flow. Choice A is true; gases diffuse per Fick's law. ' DₗCO₂ ≈ 20x DₗO₂ (600 vs. 25 mL/min/mmHg) due to solubility. Choice D is accurate; CO, diffusion-limited, doesn't equilibrate in 0.75 s (high Hb affinity). Choice E (O₂ perfusion-limited) is true at rest. N₂O's uptake depends on flow, not diffusion barrier, making C the false statement.