The lower respiratory tract consists of the larynx, the tracheobronchial system and the right and left lung. The two parts of the lung are divided from each other by the content of the mediastinum.
After the inhaled air moves through the larynx, it reaches the trachea. The trachea is a rigid, muscular tube about 11 cm long and 2.5 cm wide. Inward airflow from the trachea then branches off to the two main bronchi. Deeper in the lungs, each bronchus divides into secondary and tertiary bronchi, which continue to branch to smaller airways called the bronchioli. No cartilage is found in the bronchioli, and therefore they are subject to constriction and obstruction, as during an asthma attack. The bronchioli end in air sacs called the alveoli. Here gas exchange occurs – carbon dioxide from the blood is exchanged for oxygen from the alveoli.