The lower respiratory tract or lower airway consists of the trachea, bronchi (primary, secondary and tertiary), bronchioles (including terminal and respiratory), and lungs (including alveoli). It also sometimes includes the larynx. The lower respiratory tract is also called the respiratory tree or tracheobronchial tree, to describe the branching structure of airways supplying air to the lungs, and includes the trachea, bronchi and bronchioles. The lower airway consists of the following:

• trachea
• main bronchus (diameter approximately 1 – 1.4 cm in adults)
• lobar bronchus (diameter approximately 1 cm)
• segmental bronchus (diameter 4.5 to 13 mm)
• subsegmental bronchus (diameter 1 to 6 mm)
• conducting bronchiole
• terminal bronchiole
• respiratory bronchiole
• alveolar duct
• alveolar sac
• alveolus