Q.Compared to O2, diffusion rate of CO2 through the diffusion membrane per unit difference in partial pressure is much higher. Explain.

Exchange Of Gases

Why Oxygen Moves Into Blood and Carbon Dioxide Moves Out

Picture a crowded room: when a door to an empty hallway opens, people naturally drift toward the emptier space, not because anyone pushes them, but because there is more room there. Gases behave the same way — they move from where they are concentrated to where they are less concentrated, a movement called diffusion. Air freshly breathed into the lungs is rich in oxygen; the blood arriving at the lungs from the body is poor in oxygen and rich in carbon dioxide. So oxygen diffuses from air into blood, and carbon dioxide diffuses from blood into air.

In a mixture like air, each gas behaves as though it alone occupies the space, driven by its own partial pressure — the pressure that gas alone would exert if it filled the whole volume. The rate at which a gas diffuses depends on the difference in its partial pressure across the barrier, how soluble it is, and how large and thin the exchange surface is.

The Alveolar–Capillary Barrier

Exchange happens across an extremely thin membrane where an alveolus and a capillary lie side by side, each wall only one cell thick and fused together — a total barrier of about 0.5 micrometres. It consists of a thin fluid lining inside the alveolus, the alveolar epithelial cell, a fused basement membrane, and the capillary endothelial cell.

The Pressure Gradients In Action

Typical partial pressures at sea level, in mm Hg:

Oxygen has a large gradient of 64 mm Hg and moves quickly into blood. Carbon dioxide has a much smaller gradient of only 5 mm Hg, yet diffuses just as effectively, because it is roughly 20–25 times more soluble than oxygen in the fluid lining the alveolus — solubility compensates for the smaller pressure difference.

The Journey of a Gas Molecule …