Regarding the work of breathing during quiet inspiration:

Correct Answer: B

Rationale: viscous resistance (tissue friction) is ≈7% of quiet breathing work. Choice A is close; elastic work (lung/chest recoil) is 65-80%, but 80% is high. Choice C is reasonable; airway resistance is 10-20%, ≈13% in normals. Choice D is false; the plot shows hysteresis (loop), not a straight line, due to elastic/resistive differences. Choice E is true; total system work exceeds lung-only work (chest wall adds). In quiet breathing (500 ml TV), total work is ≈0.5 J, with elastic work dominant, viscous minor, and airway moderate. B's precision reflects tissue drag's small role, aligning with physiological data.

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: B

Rationale: medium-sized bronchi (2-4 mm diameter) are the major resistance site due to turbulent flow and peak cumulative resistance before total cross-sectional area rises. Choice A (segmental) and Choice D (large bronchioles) have less resistance due to size or area. Choice C (small bronchi) contributes less individually. Choice E (terminal bronchioles) has high total area, minimizing resistance. Poiseuille's law and airway branching show resistance peaks in medium bronchi, where radius narrows significantly, making B the primary site.

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.