The respiratory system is derived from which germ layer?

Intercostal muscles are found in ...........

Which structure opens into the alveolar sac?

Select the correct statements about alveoli from the following.
$i$. Air sac of each terminal bronchiole shows $10$ to $12$ alveoli.
$ii$. Each alveolus is surrounded by a network of capillaries.
$iii$. They are lined by ciliated pseudostratified epithelium.
$iv$. They provide surface area for exchange of gases.
$v$. There are about $700$ million alveoli in the lungs.

Which structure in the human respiratory system is involved in the conditioning of air?

Which membrane covers the lungs and provides protection?