The lungs
The trachea, bronchi and bronchioles are airways that allow passage of air into the lungs and out again. To be effective, these airways must meet certain requirements:
- The larger airways must be large enough to allow sufficient air to flow without obstruction
- They must also divide into smaller airways to deliver air to all the alveoli
- The airways must be strong enough to prevent the collapsing when the air pressure inside is low (this low pressure occurs during inhalation)
- They must be flexible, to allow movement
- They must also be able to stretch and recoil.
The trachea, bronchi and bronchioles
The only difference between the trachea and bronchi is their size. The bronchi are narrower than the trachea.
- Much of the wall consists of cartilage
- The cartilage is in the form of incomplete rings or C-rings in the trachea, but is less regular in the bronchi
- On the inside surface of the cartilage is a layer of glandular tissue, connective tissue, elastic fibres, smooth muscle and blood vessels
- The inner lining is an epithelium layer that has two types of cell. Most of the cells have cilia. This is called ciliated epithelium and among the ciliated cells are goblet cells.
Bronchioles are narrower still than the bronchi. Larger bronchioles may contain some cartilage, but smaller ones do not. The walls of the bronchioles are made of smooth muscle and elastic fibres.
At the ends of the smaller bronchioles are clusters of alveoli which are specially adapted for gas exchange.
At the ends of the smaller bronchioles are clusters of alveoli which are specially adapted for gas exchange.