Question

Which of the following is responsible for the movement of oxygen molecules from the alveoli into the blood in the pulmonary capillaries?

• A. Active transport
• B. Filtration
• C. Passive diffusion
• D. Facilitated diffusion

Hint:

Respiratory Gas Exchange: Both $\text{O}_2$ and $\text{CO}_2$ cross the alveolar-capillary barrier exclusively by Simple Passive Diffusion (Fick's Law).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Pulmonary alveolar-capillary gas exchange is governed purely by passive Fickian diffusion driven by partial pressure gradients ($\Delta P$).
Key Formula or Approach:
\[ \dot{V}_{\text{O}_2} = \frac{A \cdot D \cdot (P_{A\text{O}_2} - P_{v\text{O}_2})}{T} \quad \text{[Fick's Law of Diffusion]} \]

Step 2: Detailed Explanation:
In respiratory physiology:
- Oxygen transport across the thin blood-air respiratory membrane (alveolar epithelium, fused basement membranes, and capillary endothelium, thickness $< 0.5\;\mu\text{m}$) occurs entirely by Simple Passive Diffusion.
- It is driven downhill by the steep partial pressure gradient between alveolar air ($P_{A\text{O}_2} \approx 104\text{ mmHg}$) and deoxygenated mixed venous blood in pulmonary capillaries ($P_{v\text{O}_2} \approx 40\text{ mmHg}$), requiring no carrier proteins or metabolic energy.

Step 3: Final Answer:
Hence, oxygen movement occurs via Passive diffusion, corresponding to option (C).