In humans,where does the exchange of gases take place?

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

Name the primary site of exchange of gases in our body?

The partial pressure of $O_{2}$ and $CO_{2}$ in atmospheric air as compared to those in alveolar air is:

Gas	Alveolar Air $(X/A)$	Atmospheric Air $(Y/B)$
$pO_{2}$	$X$	$Y$
$pCO_{2}$	$A$	$B$

Partial pressure (in $mm \ Hg$) of oxygen and carbon dioxide at different parts involved in diffusion in comparison to those in the atmosphere. Identify $A$ and $B$ and choose the correct option accordingly.

Gas	Atmospheric Air	Alveoli
$O_{2}$	$159$	$A$
$CO_{2}$	$0.3$	$B$

The blood leaving the lungs has all its haemoglobin oxygenated. Why does it give up oxygen to the tissues?