Gas Exchange During Shallow Breathing at Rest

Steps:

• Shallow, slow breathing limits fresh air to alveoli, reducing oxygen concentration there.
• This decreases the concentration gradient for oxygen diffusion from alveoli to blood.
• At rest, lower metabolic demand means slower oxygen use, but ventilation limits diffusion rate.
• During activity, deeper breathing increases alveolar oxygen, raising diffusion rate.

Why D is correct:

• Fick's law of diffusion states rate is proportional to concentration gradient; rest reduces alveolar O2 gradient versus activity.

Why the others are wrong:

• A: Pulmonary vein blood has lower CO2 than artery after gas exchange removes it.
• B: CO2 exits blood by passive diffusion down its gradient, not active transport.
• C: Alveolar air has higher O2 concentration than deoxygenated blood in pulmonary artery.

Final answer: D