Which surface of a plant leaf is better adapted for gas exchange?

The number of $RBC$ in a man increases if he lives at a higher altitude because:

People living at sea level have around $5$ million $RBC$ per cubic millimeter of their blood,whereas those living at an altitude of $5400$ metres have around $8$ million. This is because at high altitude:

The rate of diffusion of gases can be affected by $-$

In the given diagram representing the partial pressure of gases in the systemic circulation,what are the partial pressures of $CO_2$ at $P$ and $Q$ respectively? (Assuming $P$ is deoxygenated blood entering the lungs and $Q$ is oxygenated blood leaving the lungs)

What are the layers encountered by $O_2$ as it passes through the diffusion membrane from the start to the end?