The exchange of gases in the alveoli of the lungs takes place by

Through which process is oxygen transported from the lungs to the tissues?

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:

Explain: Diffusion of $O_2$ from alveoli to tissues and of $CO_2$ from tissues to alveoli.

Given below are two statements.
Statement $I$: Respiratory membrane is composed of two layers of squamous epithelia,each resting on a thick basement membrane.
Statement $II$: The diffusion of a respiratory gas takes place from lower partial pressure to higher partial pressure of the same.
In the light of above statements,choose the most appropriate answer from the options given below:

Ascent of high mountains may cause altitude sickness in men. The prime cause of this is: