The trunk: trachea and the first split
Below the larynx, the trachea (windpipe) runs down the front of the neck and into the chest. It is held open by C-shaped rings of cartilage — stiff enough to never collapse, with a soft gap at the back so a swallowed bite of food can bulge past in the esophagus behind it. At its lower end the trachea forks into the right and left main bronchi. The internal ridge at that fork is the carina, a sharp keel that helps direct airflow and is a key landmark during bronchoscopy.
Branch after branch after branch
From each main bronchus the tree keeps dividing — into lobar bronchi, then segmental bronchi, then ever-smaller branches. Around 23 generations of splitting in total. As the tubes shrink they lose their cartilage and become bronchioles, whose walls are mostly smooth muscle. The last purely conducting tube is the terminal bronchiole. Beyond it, the respiratory bronchiole starts sprouting the first alveoli in its walls — this is the handoff from conducting to exchanging.
Everything fed by one terminal bronchiole — its respiratory bronchioles, alveolar ducts, and alveoli — is called an acinus. The acinus is the lung's functional unit: the smallest piece that does the whole job of bringing air to the gas-exchange surface.
Why small tubes punch above their weight
It seems backwards, but the small bronchioles offer surprisingly little airway resistance — because there are so many of them in parallel, their combined cross-section is huge. Most resistance in a healthy person actually sits in the medium-sized bronchi. But the bronchioles have no cartilage to hold them open, so when their muscle tightens or their walls swell, they narrow fast. That is exactly what happens in asthma and COPD, where small airways are the battlefield.
Airway tree, in brief:
Trachea
-> Right & Left main bronchi (split at the carina)
-> Lobar bronchi (one per lung lobe)
-> Segmental bronchi
-> ... ~ smaller bronchi
-> Bronchioles (no cartilage)
-> Terminal bronchiole <- last conducting tube
-> Respiratory bronchiole <- first alveoli appear
-> Alveolar ducts -> Alveoli
Conducting zone = nose ... terminal bronchiole (no gas exchange)
Respiratory zone = respiratory bronchiole onward (gas exchange)
One acinus = everything past a single terminal bronchiole