Trachea

The air then passes into the trachea (also known as the windpipe). It is formed by C-shaped rings of cartilage and runs from the level of C6 or the 6th cervical vertebrae and terminates at the level of T4 or the 4th thoracic vertebrae.

The trachea branches into the right and left main bronchi at the carina.

Bronchial Tree

From the trachea, the air moves to the right and left bronchi (also known as primary bronchi) which enter the lungs at the hilum.
The air continues to move through the bronchi as they continue branching into narrower bronchi (also known as secondary bronchi) which eventually branch into even narrower segmental bronchi (also known as tertiary bronchi). These tertiary bronchi then branch off into bronchioles which are no longer supported by cartilage.

The walls of bronchioles are lined with smooth muscles that help regulate the passage of the air.

Finally, the bronchioles divide into terminal bronchioles and then to respiratory bronchioles, signalling the movement of air into the next portion of the respiratory system.

Alveoli

Alveoli are located sparsely in the respiratory bronchioles, they line the walls of the alveolar ducts and appear in larger numbers in the alveolar sacs.

The respiratory bronchioles are connected to the alveolar ducts which in turn are deeply lined with the alveoli.

Each alveolar duct opens up into numerous alveolar sacs.

The alveoli which line the alveolar sacs consist of a respiratory membrane which is the site of gaseous exchange. This respiratory membrane has a layer of pulmonary surfactant fluid that functions to:

1. Increase pulmonary compliance

2. Prevent the collapse of the lungs

Alveoli are the functional unit or the parenchyma of the lungs and the alveolar membrane is responsible for gas exchange.

Alveolar walls are one cell thick and are surrounded by a rich capillary network which promotes the rapid diffusion of oxygen and carbon dioxide.