After inspiration commences the:

Correct Answer: A

Rationale: As inspiration begins, intrapleural pressure and intrapulmonary pressure both fall (choice A). Intrapleural pressure drops (e.g., from -5 to -7.5 cmHâ‚‚O) as the diaphragm contracts, expanding the thoracic cavity and creating negative pressure. This pulls the lungs outward, reducing intrapulmonary (alveolar) pressure below atmospheric (e.g., -1 mmHg), driving air in. Choice B is wrong; intrapulmonary pressure doesn't rise it falls to initiate airflow. ' venous return increases with lower intrapleural pressure, aiding thoracic pump action. Choice D is false; intrapulmonary pressure doesn't rise, and alveolar volume increases immediately as air enters. Choice E's venous return decrease contradicts physiology. The coordinated pressure drop in both spaces is essential for lung expansion and ventilation, making A the correct description of early inspiration dynamics.

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: 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.