What determines whether a respiratory system structure is part of the respiratory zone, and not part of the conducting zone?

answer: blah

explanation:

bc blah

the fossil record provides evidence of a common ancestor to many species.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.