This photo shows a cross section through one lung of a fetal pig.
In this view, you can see a primary bronchus (yellow circle) and two secondary bronchi (red circles) in cross section.
Air from the trachea enters the bronchi, which split into smaller and smaller bronchi and then bronchioles. The bronchioles end in alveoli -- microscopic sacs whose "walls" are exchange epithelium.
Next: The bronchi
Back to: Respiratory system overview