The Tracheobronchial Tree

This follows the page describing the basic anatomy of the lower respiratory tract.

The term "tracheobronchial tree" or "respiratory tree" refers to the structures of bronchi and bronchioles that terminate with the alveolar ducts, sacs, and, finally, alveoli - that are contained within the lungs. These are the structures through which air passes into the body (usually through the nose/mouth then the trachea). Therefore these structures are also referred to as "airways".

This pages includes two diagrams of the airways - the main structure of the tracheobroncial tree being shown immediately below, and a second diagram showing the alveoli in further detail further down the page.

Larger structures

The trachea divides to form the right- and left- primary bronchi (as shown). Each of these divide into lobar bronchi - which supply air to each of the lobes of the lung.

The lobar bronchi divide into segmental bronchi - which supply air to areas of the lung that are called bronchoplumonary segments.
Bronchopulmonary segments are functionally and anatomically distinct from each other - which matters because a segment of diseased lung can be removed surgically without adversely affecting the rest of the lung.

Areas of tracheobronchial tree furthest from the trachea are collectively called the "distal respiratory tree".

Above: Anterior View of the Tracheobronchial Tree

Distal Respiratory Tree
(lower airways)

As shown above, the finest (narrowest) of the bronchial air tubes are called
"terminal bronchioles".
These lead to "respiratory bronchioles" which are even smaller tubes whose structure is different from the terminal bronchioles.

Respiratory bronchioles are lined by ciliated cuboidal epithelium surrounded by smooth muscle. The respiratory bronchioles are covered by small "air cells" called alveoli. Alveolar ducts connects alveoli to the respiratory bronchiole to which they are attached.

Respiratory bronchioles and alveolar ducts occupy very similar positions on diagrams but are distinguished physically by the differences between the structure of their walls and the tissues that line them. E.g. respiratory bronchioles are lined with simple ciliated cuboidal epithelium and Clara cells whereas alveolar ducts are lined with flat nonciliated epithelium.



	Above: Distal Respiratory Tree (Gas Exchange Region)

	All of the alveoli are covered by fine blood capillaries as shown in red for the top aleveolar sace (above). Others are shown without the capillary network for clarity of illustration of the alveoli. The area shaded yellow is a cut-away section to illustrate that the alveoli are not many closed spheres but, rather, are many microscopic blind-ending air pouches. Each individual alveolus opens into a larger sac (one of many such alveoli sacs, each having many individual alveoli), that is connected to its terminal bronchiole via an alveoli duct. Also note the alveoli-capillary membrane which seperates the air inside the alveolus from the blood-carrying capillary on the outside of the alveolus. This is the membrane through which the gases oxygen and carbon-dioxide are exchanged during the breathing process (internal respiration).

Re-cap: Structure through the airways

1.	Trachea
2.	Bronchus (Right- or Left- Primary Bronchus)
3.	Lobar Bronchus
4.	Segmental Bronchus
5.	Bronchus
6.	Bronchiole
7.	Terminal Bronciole
8.	Respiratory Bronchiole
9.	Alveolar Duct
10.	Alveolar Sac / Alveolus

Next: Also see the pages about:
External Respiration and Internal Respiration.




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The Tracheobronchial Tree

Larger structures

Distal Respiratory Tree (lower airways)

Distal Respiratory Tree
(lower airways)