What is the direction of the concentration gradient for $CO_2$ and $O_2$ during gas exchange?
$CO_2$ gradient $\quad$ $O_2$ gradient

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:

Almost the same $pCO_{2}$ in humans is found in

What is the primary reason why mountaineers experience difficulty at high altitudes?

The given figure shows a small part of the human lung where gas exchange occurs. Identify the part $A, B, C,$ or $D$ correctly with its function from the given options.

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