Regarding pulmonary blood flow:

Correct Answer: C

Rationale: regional pulmonary blood flow differences (apex to base) arise from hydrostatic pressure gradients (West zones). Choice A is false; increased arterial pressure (Pa) reduces resistance via vessel distension. Choice B is true; hypoxia (low POâ‚‚) causes vasoconstriction, but isn't the question's focus. Choice D is wrong; in Zone 1 (apex), PA > Pa > Pv, stopping flow. Gravity increases Pa and Pv from apex (Zone 1, minimal flow) to base (Zone 3, high flow), with resistance adjusting inversely. C accurately describes this vertical distribution.

Correct Answer: C

Rationale: extra-alveolar vessel caliber (e.g., arterioles, venules) is primarily determined by lung volume, expanding with inflation (radial traction). Choice A affects alveolar capillaries (Zone 2/3). Choice B (artery pressure) influences flow, not caliber directly. Choice D (output) and E (fluid) are secondary. As volume rises (e.g., FRC to TLC), vessel resistance drops, distinguishing C as the mechanical driver.

Correct Answer: A

Rationale: HbS shifts the O₂ curve right (lower affinity, P₅₀ up), not left, due to altered structure (valine for glutamic acid). Choice B is true (Bohr effect). Choice C is correct (CO affinity ≈240x O₂). Choice D is accurate; COHb left-shifts remaining Hb's curve. HbS's sickling enhances O₂ unloading, contradicting A's left shift, making it false.

Correct Answer: B

Rationale: E. coli endotoxin causes pulmonary vasoconstriction (e.g., sepsis-induced hypertension), not venodilatation. Choice A is true; apex perfusion is lower (gravity). ' flow rises 3-6x (5 to 15-30 L/min). Choice D is accurate; resistance increases at low volumes (vessel collapse). Choice E (Zone 2) is true. Endotoxin triggers thromboxane/prostaglandins, constricting vessels, contradicting B's dilation, making it the error.

Correct Answer: A

Rationale: exercise does not increase PaCO₂ it remains ≈40 mmHg as ventilation matches CO₂ production (e.g., 0.2 to 4 L/min). Choice B is true; PAO₂ rises slightly (e.g., 100 to 110 mmHg) with hyperventilation. ' PaCO₂ stays stable. Choice D is accurate; PaO₂ holds at ≈95-100 mmHg in healthy lungs. Choice E (increased PvCO₂) is true (e.g., 46 to 50 mmHg). Exercise boosts V̇E (e.g., 6 to 120 L/min), preventing PaCO₂ rise, making A the exception in normal physiology.