Which is CORRECT?

Correct Answer: C

Rationale: automatic breathing's reciprocal innervation (inspiration vs. expiration) relies on descending medullary pathways (e.g., dorsal/ventral respiratory groups), not just spinal reflexes. Choice A is false; voluntary control stems from the cortex, not pons/medulla (automatic centers). Choice B is wrong; automaticity uses reticulospinal tracts, not corticospinal (voluntary). ' external intercostals aid inspiration, not expiration. Choice E (no phrenic output in expiration) is true in quiet breathing but not absolute. The pre-Bötzinger complex drives rhythm, coordinating via descending signals to inhibit expiratory neurons during inspiration. This supraspinal control ensures smooth cycling, making C the accurate depiction of automatic respiratory neural integration.

Correct Answer: D

Rationale: mixed venous PO₂ (PvO₂) is ≈50 mmHg on 100% O₂. With FiO₂ = 1.0, PaO₂ rises to ≈650 mmHg (PIO₂ = 760 - 47 ≈ 713 mmHg, minus A-a gradient). O₂ content increases (dissolved O₂ ≈ 2 ml/100 ml + Hb-bound), but tissues extract ≈5 ml/100 ml O₂, dropping PvO₂ to 50-55 mmHg (SaO₂ near 100%). Choice A (40 mmHg) is normal air breathing. Choice B (713 mmHg) is arterial, not venous. Choice C (650 mmHg) overestimates venous saturation. D reflects the balance of high arterial O₂ and tissue uptake.

Correct Answer: A

Rationale: RQ nears 1.0 in exercise (e.g., 0.8 to 1.2) as metabolism shifts. Choice B is true but incomplete; PIO₂ = FiO₂ · (PB - PH₂O). Choice C is false; PACO₂ ≈ PaCO₂ (≈40 mmHg) in normals. Choice D is wrong; PAO₂ ≈ 100 mmHg (150 - 40/0.8), not 150. Exercise increases VCO₂ relative to VO₂, raising RQ, especially anaerobically. The equation (PAO₂ = PIO₂ - PACO₂/RQ) adjusts dynamically, making A the accurate statement.

Correct Answer: A

Rationale: central chemoreceptors (medulla) dominate minute-by-minute ventilation, sensing CSF pH (CO₂-driven, 70-80% of response). Choice B (carotid) and C (aortic) contribute <20% (O₂, CO₂, pH), critical in hypoxia. Choice D (stretch) fine-tunes via Hering-Breuer reflex. Central receptors adjust breathing rate/depth to maintain PaCO₂ ≈ 40 mmHg, responding within seconds to CO₂ shifts, making A the primary controller.

Correct Answer: C

Rationale: high altitude causes pulmonary vasoconstriction (hypoxic response), not vasodilation. Choice A is true; 2,3-DPG shifts the curve right. ' angiogenesis increases capillaries. Choice D is accurate; polycythemia boosts Hb. Choice E (breathing capacity) rises. Vasoconstriction matches V/Q, countering hypoxia, making C the exception.