The structure of the bronchial tree

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Semey State Medical University

SIW

Theme: “The structure of the bronchial tree”.

Prepared by: Yerzhan L., 341-GMF

Checked by: Kozhanova S.K.

Semey 2015

The Tracheobronchial Tree

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".

bronchial tree trachea

Trachea

The trachea, also called the windpipe, is part of the passageway that supplies air to the lungs. Any prolonged blockage, even for a few minutes, can cause death.

The trachea is about 4.5 inches long and 1 inch in diameter, and is comprised of smooth muscle and several c-shaped rings of cartilage. The rings of cartilage provide stability and help to prevent the trachea from collapsing and blocking off the airways. The trachea extends from the neck and divides into two main bronchi.

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".

Bronchi

Structurally similar to the trachea, the two primary bronchi are located inside the lungs. The right bronchus is slightly larger than the left one. Because of this, foreign objects breathed into the lungs often end up in the right bronchus. The bronchi are lined with the same type of mucus that lines the rest of the respiratory tract.

Deeper into the lungs, each bronchus is further divided into five smaller, secondary bronchi, which provide air to the lobes of the lungs. The secondary bronchi continue to branch off to form the tertiary bronchi, which are further divided into terminal bronchioles. There are as many as 30,000 tiny bronchioles in each lung. They lead to the alveoli by way of alveolar ducts.

Bronchial tree

Together, the trachea and the two primary bronchi are referred to as the bronchial tree. At the end of the bronchial tree lie the alveolar ducts, the alveolar sacs, and the alveoli. 

The tubes that make up the bronchial tree perform the same function as the trachea: they distribute air to the lungs. The alveoli are responsible for the primary function of the lungs, which is exchanging carbon dioxide and oxygen. 

A layer of protective mucus, called a mucus blanket, covers a large portion of the membrane lining the bronchial tree. The mucus is an important air purifier.

The average adult produces about 125 milliliters of mucus daily, which is slightly more than half a cup. Microscopic, hair-like cilia move the cleansing mucus up to the pharynx--part of the throat between the mouth and esophagus--from the lower part of the bronchial tree. Cigarette smoke paralyzes the cilia, which allows mucus to accumulate and leads to what is called smoker's cough.

The bronchi are lined with respiratory epithelium and have a similar overall structure to the trachea, with two exceptions:

A smooth muscle layer intervenes between the submucosa and the cartilage

The cartilagenous rings are smaller and less complete, eventually disappearing when the bronchi become bronchioles.

The bronchioles are lined by a simple cuboidal epithelium, and their wall is much thinner than the bronchi. Their smooth muscle component is relatively larger.

Relations

The right main bronchus passes posterior to right pulmonary artery, inferior vena cava and ascending aorta. The azygos vein passes over its superior surface to enter the inferior vena cava. The oesophagus is posterior.

The left main bronchus passes posterior to the left pulmonary artery, but anterior to the descending thoracic aorta.

The vagus nerve passes anterior and posterior to the main bronchi, but mostly posterior as the pulmonary plexus.

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.

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 notmany 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).

Conclusion

The bronchial tree is an essential part of the respiratory system. It consists of several interacting structures, such as the bronchi, bronchioles, and alveoli. These structures work together to provide a network system between the lungs and the trachea. Without this system, a person could not breathe properly.

The bronchial tubes are the largest parts of this structure. There is one bronchial tube connected to each lung. The connection occurs in an upper portion of the lung known as the helium.

Although the right and left bronchial tubes perform the same tasks, they are not identical. The right bronchial tube, for example, is shorter than the left. It is also wider than its counterpart.

The primary bronchi also branch off, forming two smaller bronchial tubes known as the lobar bronchi, or secondary bronchi. There are three lobar bronchi on the right side and two on the left. The bronchiole tree's parts continue reducing in size as these secondary bronchi become smaller tubes known as the bronchioles.

As roots branch out in the ground, the bronchioles branch out and cover the surface of the lungs. These muscular structures expand and contract, controlling the exchange of gases with the alveoli. The alveoli are tiny structures composed of ducts and air sacs. They allow the exchange of the gases in the blood. Due to these tiny structures, carbon dioxide can be transported out and oxygen can be processed in.

The bronchial tree provides a system for the trachea to service the lungs. It is important to note, however, that like the trachea, it does not include the lungs. The bronchial tree begins with the primary bronchi and ends with the alveoli.

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