Which of the following will decrease diffusion?

Correct Answer: D

Rationale: Diffusion of gases like O2 and CO2 across the alveolar-capillary membrane follows Fick's law: Rate = (A × D × ΔP) / d. Decreased surface area (A), as in emphysema, reduces the available exchange area, lowering diffusion. Increased fluid in the lung (e.g., pulmonary edema) increases diffusion distance (d), as fluid thickens the barrier, impeding gas transfer and often adding proteinaceous debris that further slows diffusion. Decreased pressure coefficient' likely intends partial pressure gradient (ΔP); reducing this (e.g., via hypoventilation) weakens the driving force for diffusion. All these factors surface area, distance, and gradient when altered as described, decrease diffusion rate. The diffusion coefficient (D) isn't directly mentioned, but the combined impact of the listed changes aligns with clinical scenarios (e.g., edema causing hypoxemia). Since each independently and collectively impairs diffusion, all contribute to a reduced gas exchange efficiency, critical for oxygenation and CO2 removal.

Correct Answer: A

Rationale: During mild exercise, pulmonary gas exchange adapts to increased O2 demand and CO2 production. CO2 diffuses ~20 times faster than O2 across the alveolar-capillary membrane due to its higher solubility (0.51 vs. 0.024 ml/mmHg/L), despite a slightly higher molecular weight (44 vs. 32), per Fick's law (D ∝ solubility / √MW) making it cross easier, a true statement. Diffusing capacity (DL) for O2 is less than for CO2 normally, and while exercise increases DL for both (recruiting capillaries), CO2's advantage persists, not O2's, and molecular weight is secondary to solubility. Capillary equilibrium length shortens for O2 and CO2 as blood flow rises, but this is nuanced and not uniquely true without context. Arterial blood gases (ABGs) remain normal in healthy individuals during mild exercise (e.g., PaO2 ~100 mmHg, PaCO2 ~40 mmHg), as ventilation matches perfusion. CO2's easier diffusion is the standout truth, rooted in its physicochemical properties, critical for efficient CO2 elimination.

Correct Answer: B

Rationale: Total ventilation (VE) = VT × RR; alveolar ventilation (VA) = (VT - VD) × RR. Initially, T and V have VT = 1000 ml, VD = 200 ml, RR = 20/min. VE = 1000 × 20 = 20 L/min; VA = (1000 - 200) × 20 = 800 × 20 = 16 L/min. For T: VT doubles to 2000 ml, RR halves to 10/min. VE = 2000 × 10 = 20 L/min (constant); VA = (2000 - 200) × 10 = 1800 × 10 = 18 L/min (increases). For V: VT halves to 500 ml, RR doubles to 40/min. VE = 500 × 40 = 20 L/min (constant); VA = (500 - 200) × 40 = 300 × 40 = 12 L/min (decreases). T's larger VT boosts VA despite lower RR, as more air exceeds VD. V's smaller VT reduces VA, as dead space consumes a larger fraction per breath despite higher RR. Option B (T: VE constant, VA increases; V: VE constant, VA decreases) matches, reflecting how VT impacts VA efficiency at fixed VE.

Correct Answer: B

Rationale: Severe respiratory muscle weakness (e.g., in myasthenia gravis) impairs ventilation by weakening inspiratory and expiratory muscles. Tidal volume (VT, ~500 ml normally) decreases due to limited inspiratory force, reducing breath size. Vital capacity (VC, ~4-5 L) drops as maximal inhalation and exhalation are compromised. Oxyhemoglobin saturation falls (e.g., from 95-100% to <90%) as hypoventilation lowers PaO2, causing hypoxemia. Arterial pH may decrease (acidosis) if CO2 retention raises PCO2, but this isn't specified as above normal. However, PCO2 itself (normal 35-45 mmHg) rises above normal (e.g., 50-60 mmHg) due to inadequate CO2 expulsion, a direct result of weak ventilation. Though not listed, if B intended PCO2 (a common mix-up), it fits; otherwise, none are above normal' assuming intent, PCO2's rise is the key abnormality, reflecting ventilatory failure's impact on gas exchange.

Correct Answer: A

Rationale: Total lung capacity (TLC) is the maximum air lungs hold after maximal inspiration, summing residual volume (RV, ~1-1.5 L), expiratory reserve volume (ERV, ~1-1.5 L), tidal volume (VT, ~0.5 L), and inspiratory reserve volume (IRV, ~2-3 L). In adults, TLC averages ~6 L (5-7 L, varying by sex, age, size), per standard physiology (e.g., Guyton). Two liters approximates FRC (~2.5-3 L), the resting volume. Four liters nears vital capacity (VC, ~4-5 L), excluding RV. Nine liters exceeds typical capacity, possibly hyperinflation. Six liters aligns with spirometry plus RV (e.g., helium dilution), reflecting full lung expansion in health, making it the best approximation for a normal human, widely validated across respiratory studies.