Which is INCORRECT?

Correct Answer: C

Rationale: Choice C is incorrect because bronchioles and terminal bronchioles lack cartilage; cartilage is present in bronchi, diminishing by segmental levels. Choice A is true; the conducting zone includes 16 generations (trachea to terminal bronchioles), excluding gas exchange zones. Choice B is mostly correct; terminal bronchioles have some cilia, transitioning to club cells, though less than proximal airways. Choice D is false but less definitively; smooth muscle increases distally but isn't ‘greatest' in terminal bronchioles compared to total airway mass. Cartilage supports larger airways, replaced by smooth muscle and elastic fibers in bronchioles for flexibility and recoil. Terminal bronchioles, the last conducting segment, regulate airflow via muscle, not structural rigidity. C's assertion of cartilage in these walls contradicts airway histology, making it the most incorrect statement physiologically.

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.

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.