Explain: $(1)$ Exchange of gases by alveolus surface. $(2)$ Exchange of gases in tissues.

(N/A) $(1)$ Exchange of gases by alveolus surface:
The partial pressure of $O_{2}$ $(pO_{2})$ in deoxygenated blood reaching the alveoli is $40 \ mm \ Hg$,and the partial pressure of $CO_{2}$ $(pCO_{2})$ is $45 \ mm \ Hg$.
In the alveoli,the partial pressure of $O_{2}$ is $104 \ mm \ Hg$ and $pCO_{2}$ is $40 \ mm \ Hg$.
Due to this pressure gradient,$O_{2}$ diffuses from the alveoli into the blood,and $CO_{2}$ diffuses from the blood into the alveoli.
$(2)$ Exchange of gases in tissues:
In tissues,the exchange of gases occurs between the blood and the tissue cells via the interstitial fluid (tissue fluid).
Oxygenated blood reaching the tissues has a $pO_{2}$ of $95-100 \ mm \ Hg$,while the $pO_{2}$ in the metabolically active tissue cells is $40 \ mm \ Hg$.
The $pCO_{2}$ in the tissues is $45 \ mm \ Hg$,while in the blood,it is $40 \ mm \ Hg$.
Due to these partial pressure gradients,$O_{2}$ diffuses from the blood into the tissues,and $CO_{2}$ diffuses from the tissues into the blood.