The lungs receive air from the bronchi (sing: bronchus). As the bronchi enter the lungs, they branch into smaller and smaller segments, which are referred to as bronchioles. The smallest bronchioles are the respiratory_bronchioles, which connect to alveolar_ducts. Alveolar ducts, in turn, connect to smaller structures called alveolar_sacs. These then connect to individual air sacs termed alveoli.