The divisions of the bronchial tree are bronchus, bronchioles, respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli.

The left lung has two lobar bronchi and they both have five divisions.

The right lung has three lobar bronchi. The right upper lobe has three divisions; the right middle lobe has two divisions and the right lower lobe, five divisions.

The left main bronchus passes under the aortic arch. It is posterior to the left pulmonary artery and is anterior to the oesophagus, aortic arch and thoracic duct.

The right main bronchus lies above and behind the right pulmonary artery and passes under the azygous vein arch.

The airway consists of 23 generations and the respiratory bronchioles are included in the distal seven generations.

Gas exchange occurs within the respiratory bronchioles, alveolar ducts and alveoli.

The surface area of the alveoli is between 500, 000 - 1, 000, 000 cm² (or approximately 50-100 m²). The velocity of airflow is low because of the large cross-sectional area of airways.