Number Name of branches of tubes in branch Trachea The relaxation/contraction Bronchi of circular smooth muscle lining these“ airways 9NEa 8 determines how easily Bronchioles airflow can occur Terminal bronchioles 6x1 Respiratory bronchioles Most gas exchange 5x10 occurs in the alveolar sacs cON>o@a Alveolar ducts Alveolar sacs 8x106The relaxation/contraction of circular smooth muscle lining these “airways’” determines how easily airflow can occur. Most gas exchange occurs in the alveolar sacs