Which of the following substances is activated by passage through the pulmonary circulation?

Correct Answer: D

Rationale: Angiotensin I (choice D) is activated to angiotensin II by angiotensin-converting enzyme (ACE) in pulmonary capillaries, making it correct. Bradykinin (choice A) is inactivated in the lungs by ACE, not activated. Serotonin (choice B) is taken up and metabolized by pulmonary endothelium, reducing its activity. Noradrenaline (choice C) is similarly removed, not activated, via uptake and degradation. Vasopressin (choice E) passes through unchanged. The lungs' role as a metabolic filter activates angiotensin I during circulation, crucial for blood pressure regulation, while degrading vasoactive peptides like bradykinin and serotonin. This specificity of pulmonary ACE action distinguishes D as the substance activated, aligning with its physiological role in the renin-angiotensin system.

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: decreasing breathing work (e.g., assisted ventilation) doesn't reduce COâ‚‚ response it may enhance it by easing effort. Choice A (sleep), B (barbiturates), C (age), and E (athletes/divers) depress sensitivity via CNS suppression or adaptation. COâ‚‚ drives ventilation via central chemoreceptors; reduced work lowers resistance, not chemosensitivity. D stands out as the exception.

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.