Outline the structure of the trachea

Explain the functions of the trachea in respiration

Structure of bronchi

Structure of bronchiole

The trachea is a tube formed of cartilage and fibromuscular membrane, lined internally by mucosa.

Begins at the inferior border of the cricoid cartilage (C6) as a continuation of the larynx and ends by bifurcating at the carina into the right and left main stem bronchi at the level of the sternal angle (disc between T4 and T5).

Is approximately 10-12 cm in length and has 16 to 20 incomplete hyaline cartilaginous rings that open posteriorly toward the esophagus and prevent the trachea from collapsing.

Superiorly – the larynx

Inferiorly – the left and right bronchi

Anteriorly –upper part: the isthmus of the thyroid gland, lower part: the arch of the aorta and the sternum

Posteriorly – the esophagus separate the trachea from vertebral column.

Laterally – the lungs and lobes of thyroid gland

The trachea is composed of from 16 to 20 incomplete (C-shaped) rings of hyaline cartilages lying one above the other.

The free ends of these rings are supported by the trachealis muscle.

Connective tissue and involuntary muscle join the cartilages and form the posterior wall where they are incomplete.

The soft tissue posterior wall is in contact with the esophagus.

There are three layers of tissue which ‘clothe’ the cartilages of the trachea.

The outer layer. This consists of fibrous and elastic tissue and encloses the cartilages.

The middle layer. This consists of cartilages and bands of smooth muscle that wind round the trachea in a helical arrangement. There is some areolar tissue, containing blood and lymph vessels and autonomic nerves.

The inner lining. This consists of ciliated pseudostratified columnar epithelium, containing mucus-secreting goblet cells.

The combination of sweeping movements by the cilia and mucus from the goblet cells forms the functional mucociliary escalator.

The nerve supply.

Parasympathetic supply is by the recurrent laryngeal nerves and other branches of the vagi.

Sympathetic supply is by nerves from the sympathetic ganglia.

Blood circulation.

Arterial supply is mainly by the inferior thyroid and bronchial arteries

Venous return is by the inferior thyroid veins into the brachiocephalic veins

Mucociliary escalator

Support and patency

Warming, humidifying and filtering of air

Support and patency

Cough reflex.

It conducts air into the lungs.

It is made up of hyaline cartilage.

It is lined by the mucous membrane which undergoes a transition from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium.

No gas exchange takes place in this part of the lungs.

Two primary bronchi are formed when the trachea divides, about the level of the 5th thoracic vertebra.

THE RIGHT BRONCHUS

This is wider, shorter and more vertical than the left bronchus and is therefore the more likely of the two to become obstructed by an inhaled foreign body.

After entering the right lung at the hilum it divides into three branches, one to each lobe.

Each branch then subdivides into numerous smaller branches.

This is narrower than the right.

After entering the lung at the hilum it divides into two branches, one to each lobe.

Each branch then subdivides into progressively smaller tubes within the lung substance.

Within the lungs, the main(primary) bronchi branch into lobar (secondary) bronchi

There are 3 right lobar and 2 left lobar bronchi

The lobar bronchi then bifurcate into several segmental (tertiary) bronchi, each of which supplies a bronchopulmonary segment, which are subdivisions of the lung lobes, and acts as the functional unit of the lungs.

The structure of bronchi are very similar to that of the trachea

Though differences are seen in the shape of their cartilage

In the main bronchi, cartilage rings completely encircle the lumen.

However in the smaller lobar and segmental bronchi cartilage is found only in crescent shapes.

The segmental bronchi undergo further branching to form numerous smaller airways- the bronchioles

Do not contain any cartilage or mucus-secreting goblet cells.

Club cells produce a surfactant lipoprotein which is instrumental in preventing the wall of the small airways sticking together during expiration.

Conducting bronchioles, lacking gland, ends as terminal bronchioles

Terminal bronchioles branch further into respiratory bronchioles, which are distinguishable by the presence of alveoli extending from their lumens

Terminal bronchioles branch further into respiratory bronchioles, which go on to divide into 2 to 11 alveolar ducts

There are 5 or 6 alveolar sacs associated with each alveolar duct.

ALVEOLI are tiny air-filled pockets with thin walls (simple squamous epithelium) and are the site of gaseous exchange in the lungs.