Gas Exchange in the Lungs

Steps:

• Pulmonary artery carries deoxygenated blood from the body with low partial pressure of oxygen (PO2 ≈ 40 mmHg).
• In pulmonary capillaries, oxygen diffuses from alveoli (high PO2 ≈ 100 mmHg) into blood due to concentration gradient.
• This oxygenation increases blood PO2 to ≈ 100 mmHg.
• Oxygenated blood returns via pulmonary vein, creating the PO2 difference.

Why A is correct:

• Oxygen diffusion from alveoli to blood follows Fick's law, where net diffusion occurs down the partial pressure gradient, raising PO2 in pulmonary venous blood.

Why the others are wrong:

• B: Describes systemic capillaries in body tissues, not pulmonary circulation.
• C: Occurs in systemic capillaries for tissue oxygenation, not lungs.
• D: Involves CO2 diffusion from tissues to blood in systemic capillaries, irrelevant to pulmonary O2 difference.

Final answer: A