Regarding oxygen transport:

Correct Answer: D

Rationale: Choice D is correct (replacing E); normal P₅₀ is ≈26-27 mmHg, the pO₂ at 50% Hb saturation. Choice A is false; HbS has valine replacing glutamic acid (β6 mutation). Choice B is wrong; saturation is the percentage of sites with O₂, not without. ' oxygenated Hb is the R (relaxed) state, not T (tense). Choice E (27 mmHg) is true but redundant here. P₅₀ reflects Hb-O₂ affinity; 27 mmHg is standard at pH 7.4, 37°C, aligning with physiological O₂ unloading. D's accuracy makes it the correct statement.

Correct Answer: B

Rationale: 2,3-DPG levels rise at altitude (hypoxia response), shifting the curve right. Choice A is true; PCO₂ shifts it right (Bohr effect). Choice C is false; deoxy-Hb is tense (T state), not relaxed (R state). Choice D is wrong; CO's affinity is ≈240x, shifting left by reducing available Hb, not curve shape. Altitude adaptation increases 2,3-DPG, aiding O₂ unloading (P₅₀ up), making B the accurate chronic response.

Correct Answer: D

Rationale: irritant receptors, between epithelial cells, adapt rapidly to mechanical/chemical stimuli (e.g., dust, smoke), triggering cough/bronchoconstriction. Choice A (central) is chemical, not airway-based. Choice B (carotid) and C (aortic) are vascular. Choice D (stretch) adapts slowly. Irritant receptors, vagally mediated, protect airways with quick responses, distinguishing E as the correct type.

Correct Answer: C

Rationale: lower regions (base) ventilate better than upper (apex) due to gravity and compliance gradients. Choice A is true (TV ≈ 500 mL). Choice B is correct (dead space ≈ 150-160 mL). Choice D is accurate (Bohr's method). Choice E (none) is false. Base ventilation exceeds apex (e.g., 4:1 ratio upright), reflecting diaphragm motion and pleural pressure, making C the error.

Correct Answer: D

Rationale: increased carboxyhemoglobin (COHb) shifts the curve left, not right, by enhancing Hb-O₂ affinity (CO binds 240x stronger). Choice A (temperature), B (PCO₂), and C (H⁺) shift it right via the Bohr effect, aiding O₂ unloading (P₅₀ up, e.g., 26 to 30 mmHg). Choice D (DPG) also shifts right. COHb reduces available Hb, left-shifting the curve for remaining O₂, impairing tissue delivery (e.g., in poisoning), opposite to exercise adaptations. E's distinct effect makes it the exception.