Simple cuboidal epithelium is found in:

Correct Answer: B

Rationale: Simple cuboidal epithelium, a single layer of cube-shaped cells, is specialized for secretion and absorption. It lines thyroid gland follicles (B), where it secretes thyroid hormones, and other structures like kidney tubules. The intestine (A) has simple columnar epithelium for nutrient absorption, with taller cells and microvilli. The epidermis (C) is stratified squamous, designed for protection, not a single layer. The trachea (D) features pseudostratified columnar epithelium with cilia and goblet cells for mucus clearance. B is correct thyroid follicles match the description, unlike the others, which serve different functions requiring distinct epithelial types.

Correct Answer: B

Rationale: Respiratory bronchioles are lined with simple cuboidal epithelium (B), transitioning from conducting airways' pseudostratified columnar (C) to alveoli's simple squamous (A). Simple columnar (D) is in intestines, not lungs. B is correct cuboidal cells with some cilia and club cells suit gas exchange initiation, unlike others.

Correct Answer: D

Rationale: Diffusion rate, per Fick's law, depends on surface area (choice A), concentration gradient (choice B), solubility (choice C), and membrane thickness (choice E), but not directly on molecular weight of gas (choice D), making D correct. Surface area increases diffusion by providing more space for gas exchange, as seen in alveoli. The concentration gradient (partial pressure difference) drives diffusion, e.g., Oâ‚‚ from alveoli to blood. Solubility affects how easily gases dissolve in the membrane, with COâ‚‚ diffusing faster than Oâ‚‚ due to higher solubility. Thickness inversely affects rate; thinner alveolar walls enhance diffusion. Molecular weight influences diffusion speed in free gas (Graham's law), but in the lung's aqueous environment, solubility dominates over molecular weight for Oâ‚‚ and COâ‚‚ exchange across membranes. Studies show diffusion capacity correlates with solubility and pressure gradients, not molecular weight, in physiological contexts, confirming D as the factor least affecting diffusion rate here.

Correct Answer: D

Rationale: With ventilation halved (e.g., from 6 L/min to 3 L/min) and CO₂ output unchanged, arterial pCO₂ rises to 80 mmHg (choice D). Normally, pCO₂ = (V̇CO₂ × K) / V̇A, where V̇CO₂ is CO₂ production, K is a constant (≈863), and V̇A is alveolar ventilation. If pCO₂ is 40 mmHg at 6 L/min, V̇CO₂ = (40 × 6) / 863 ≈ 0.278 L/min. Halving ventilation to 3 L/min, new pCO₂ = (0.278 × 863) / 3 ≈ 80 mmHg. Choice A (50) underestimates doubling; B (60) fits a 33% drop; C (70) is arbitrary. Barbiturates depress respiratory drive, reducing V̇A without altering metabolism (V̇CO₂), doubling pCO₂ as ventilation inversely governs it. Thus, D is correct.

Correct Answer: D

Rationale: Alveolar ventilation (V̇A) = (tidal volume - dead space) × respiratory rate. With TV = 600 ml, dead space ≈ 150 ml, and rate = 10/min, V̇A = (600 - 150) × 10 = 4500 ml/min (choice D). Choice A (1000 ml) fits a lower TV or rate (e.g., 250 ml × 4). Choice B (1750 ml) suggests miscalculation (e.g., 325 ml × 5). Choice C (3000 ml) aligns with TV = 450 ml at 10/min. Choice E (6000 ml) ignores dead space (600 × 10). Dead space (150 ml) is air not reaching alveoli; only 450 ml/breath ventilates gas exchange zones. At 10 breaths, 4500 ml/min reflects effective CO₂ removal and O₂ uptake, matching physiological norms for a resting adult, confirming D.