Which of the following is true regarding $CO_2$ concentration?

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:

Which structure of the lungs is directly involved in $O_2 / CO_2$ exchange between air and blood capillary?

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:

Under normal physiological conditions,$100 \, mL$ of oxygenated blood delivers how much $CO_2$ to the alveoli?

Match the partial pressures of oxygen in different parts involved in diffusion compared to the atmosphere.

Column-$I$	Column-$II$ ($PO_2$ in $mm\, Hg$)
$P$. Atmospheric air	$I$. $159$
$Q$. Tissues	$II$. $95$
$R$. Oxygenated blood	$III$. $40$