The Tracheobronchial Tree

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

The expressions tracheobronchial tree or respiratory tree refer to the structures of bronchi and bronchioles that terminate with the alveolar ducts, sacs, and, finally, alveoli, all of which are contained within the lungs. These are the structures through which air passes into the body, usually through the nose or mouth then the trachea. These structures are also called airways.

Below are two diagrams of the airways. The first shows the main structure of the tracheobroncial tree and the second shows the alveoli in further detail.

Larger structures

Above: Anterior View of the Tracheobronchial Tree

The trachea divides to form the right- and left- primary bronchi.

Each of the right- and left- primary bronchi divide into lobar bronchi which supply air to each of the two lobes of the lungs.

The lobar bronchi divide into segmental bronchi which supply air to areas of lung called bronchoplumonary segments.

The bronchopulmonary segments are functionally and anatomically distinct from each other. This matters because a segment of diseased lung can be removed surgically without adversely affecting the rest of the lung.

The areas of tracheobronchial tree furthest from the trachea are called the distal respiratory .

Distal Respiratory Tree
(lower airways)

Above: Distal Respiratory Tree (Gas Exchange Region)

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: Respiratory bronchioles are lined with simple ciliated cuboidal epithelium and Clara cells. Alveolar ducts are lined with flat nonciliated epithelium.

All of the alveoli are covered by fine blood capillaries as shown in red for the top aleveolar sacs (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) which is connected to its terminal bronchiole via an alveoli duct. An alveoli-capillary membrane separates 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




Bronchus (Right- or Left- Primary Bronchus)


Lobar Bronchus


Segmental Bronchus






Terminal Bronciole


Respiratory Bronchiole


Alveolar Duct


Alveolar Sac / Alveolus

Next: See also notes about: external respiration and internal respiration.

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