<Li> <P> Illustration depicting respiratory system </P> </Li> <P> Illustration depicting respiratory system </P> <Li> <P> Mainstem bronchi seen branching off the trachea (labeled at top), with the left mainstem bronchi passing below the aortic arch before branching into smaller bronchi . </P> </Li> <P> Mainstem bronchi seen branching off the trachea (labeled at top), with the left mainstem bronchi passing below the aortic arch before branching into smaller bronchi . </P>

The mainstem bronchus ends at the level of the