Fill in the blank:

The opening of the windpipe is guarded by a flap called .

Fill in the blank with an appropriate word:

In human beings, exchange of gases takes place in _____(bronchi/trachea/alveoli).

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The bronchioles open into thin air sacs called _____.

Fill in the blank with the appropriate word:

The lungs are covered by a pair of membrane called the _____ (pleural/pericardium/meninges) membranes.