Which does not stimulate carotid bodies?

Correct Answer: A

Rationale: carbon monoxide (CO) poisoning doesn't stimulate carotid bodies effectively, as they sense arterial pO₂, not O₂ content. CO binds hemoglobin (COHb), reducing O₂ delivery, but PaO₂ stays normal (≈100 mmHg), masking hypoxia. Choice B (cyanide) triggers them via metabolic acidosis/hypoxia signals. Choice C (hypoxia, low pO₂) directly activates them (<60 mmHg). Choice D (hypercapnia) stimulates via pCO₂ and pH changes. Choices E (H⁺) and F (nicotine) also activate them. Located at carotid bifurcations, these chemoreceptors drive ventilation in hypoxia or acidosis. CO's failure to lower PaO₂ distinguishes A as the non-stimulant, despite tissue hypoxia.

Correct Answer: D

Rationale: Choice D is correct (replacing E); helium (low density) reduces turbulence (lower Reynolds number, Re = ρvD/μ). Choice A is false; large airways (higher velocity) are more turbulent than small (laminar). Choice B is wrong; turbulent flow scales less than linearly with pressure (not doubled). ' turbulence depends on density, not viscosity (laminar does). Low-density gases decrease Re, easing flow in obstructive disease, making D true.

Correct Answer: D

Rationale: the chloride shift moves HCO₃⁻ out of the RBC (not in) and Cl⁻ in, maintaining neutrality as CO₂ forms HCO₃⁻. Choice A is true (Fick's law: V ∝ A/T). Choice B is correct (Bohr effect: PCO₂ → H⁺ → right shift). Choice C is accurate (Henry's law). Choice E is true (Haldane effect). D reverses ion directions; HCO₃⁻ exits via band 3 exchanger, Cl⁻ enters, supporting CO₂ transport, making it the error.

Correct Answer: B

Rationale: carotid bodies contain glomus cells Type I (dopamine-rich, neurotransmitter) and Type II (support) sensing POâ‚‚, PCOâ‚‚, pH. Choice A (central) lacks glomus cells. Choice C (aortic) has similar cells but is less prominent. Choice D (stretch) is mechanical. Carotid bodies, at carotid bifurcations, use Type I cells to signal hypoxia, releasing dopamine, making B the precise match.

Correct Answer: A

Rationale: diffusion capacity (Dₗ) depends on partial pressure gradient (P1-P2) driving gas transfer (Fick's law: V = D·A·ΔP/T). Choice B (area), C (thickness), and D (constant) are factors, but Dₗ integrates them with ΔP as the flux determinant. Choice E (all) is too broad. Dₗ (e.g., 25 mL/min/mmHg for O₂) measures transfer rate per pressure difference, making A the key variable.