What Are the Trachea and Stem Bronchi Structure Functions and Diagram
The trachea is a tube about 10 to 12 cm (3.9 to 4.7 inches) long and 2 cm (0.8 inches) wide that lies beneath the larynx. It has approximately 16 to 20 horseshoe-shaped, incomplete cartilage rings that open toward the back and are embedded in dense connective tissue stiffening its wall. On the other side, the dorsal wall contains a strong layer of the transverse smooth muscle fibres, which spans the cartilage gap.
About Trachea
The interior part of the trachea is lined by a typical respiratory epithelium. Mucous glands are found in the mucosal layer, and the trachea divides into two stems (or main) bronchi, one for each lung, in an inverted Y at its lower end. The main right bronchus contains a larger diameter, which is oriented more vertically and is shorter compared to the left main bronchus. The practical consequence of this arrangement is that the foreign bodies passing beyond the larynx will slip into the right lung usually. The arrangement of the stem bronchi is very similar to that of the trachea.
Structural Design of the Airway Tree
The structure of an internal lung is largely defined by the hierarchy of the dividing airways, as well as a portion of the blood vessels penetrating the lung. The intrapulmonary airway system can be divided into three functional zones: proximal, which is solely conducting, peripheral, which is purely gas-exchanging, and an intermediate zone in between, where both act the same way. However, from a morphological standpoint, it clearly makes sense to distinguish the relatively thick-walled, purely air-conducting tubes from the branches of the airway tree, which is structurally designed to permit the gas exchange.
The below figure shows the trachea, bronchi, and bronchioles of the human airway tree.
An X-ray of human lungs representing the branching of the airway tree is given below.
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Functionally, the structural design of the airway tree is essential because the branching pattern plays a major role in determining particle deposition and airflow. In modelling the human airway tree, it is generally agreed that the airways branch as per the rules of irregular dichotomy. At the same time, a regular dichotomy means that every branch of a treelike structure gives rise to the two daughter branches of identical dimensions.
However, in irregular dichotomy, the daughter branches can greatly differ in diameter and length. The average path from the trachea to the lung periphery will be calculated using up to 24–25 generations of branches, according to the models. However, the individual paths may range from 11 - 30 generations. The transition between the respiratory and the conductive portions of an airway lies on average at the end of the 16-generation if the trachea is counted as the generation-0. The two stem bronchi, the trachea, and the bronchi, including the bronchioles, make up the conducting airways.
Their main job is to steam, moisten, and clean the inhaled air before distributing it to the lung's gas-exchanging region. A typical respiratory epithelium lines them, with multiple mucus-secreting goblet cells and ciliated cells interspersed. These ciliated cells are found further down in the airway tree, and their height, like the frequency of goblet cells, decreases as the tubes narrow. In the case of bronchioles, the goblet cells are totally replaced by the other type of secretory cells called Clara cells.
A layer of low-viscosity fluid may be applied to the epithelium, causing the cilia to beat in a rhythmic, coordinated pattern directed outward. This fluid layer is topped by a blanket of high-viscosity mucus in the wider airways. The ciliary activity often drags the mucus layer along, carrying the intercepted particles to the pharynx, where they are swallowed. This design is similar to a particle conveyor belt, and the mucociliary escalator is the name for this device.
Whereas the cartilage plates or rings provide the support for walls of the bronchi and trachea, devoid of cartilage, the walls of bronchioles gain their stability from their structural integration into the tissues of gas-exchanging type. The conductive airways of the last pure generations in the lung are said to be terminal bronchioles. The presence of cup-like outpouchings from the walls changes the airway structure significantly distally. These form some minute air chambers, and they represent the first gas-exchanging alveoli on the airway path.
The respiratory epithelium also gives way to a flat-lining layer in the alveoli, allowing for the creation of a thin air–blood barrier. The alveoli are packed so tightly along the airway after several generations (Z) of such respiratory bronchioles that an airway wall proper is missing; the airway has alveolar ducts. The airway tree's final generations come to a halt in the alveolar sacs.
The Lungs
Gross Anatomy
The lung is divided into two sections, a right lung and a left lung, which take up the majority of the intrathoracic volume. The mediastinum, which corresponds to a connective tissue space comprising the main blood vessels, heart, trachea with stem bronchi, thymus gland, and oesophagus, fills the space between these two.
FAQs on Trachea and Stem Bronchi in the Human Respiratory System
1. What is the trachea and what is its function?
The trachea, also called the windpipe, is a cartilaginous tube that connects the larynx to the stem bronchi and conducts air to and from the lungs.
- It allows the passage of air during breathing.
- It keeps the airway open with C-shaped hyaline cartilage rings.
- It filters, warms, and moistens inhaled air through its mucosal lining.
- It helps remove dust particles using the mucociliary escalator mechanism.
2. Where is the trachea located in the human body?
The trachea is located in the neck and upper thorax, extending from the larynx to the level of the fifth thoracic vertebra (T5), where it divides into the right and left stem bronchi.
- It lies anterior to the esophagus.
- It begins at the level of the C6 vertebra.
- Its lower end forms the carina, the point of bifurcation.
3. What are the stem bronchi?
The stem bronchi, also called primary bronchi, are the two main airways that branch from the trachea and enter each lung.
- The right stem bronchus supplies the right lung.
- The left stem bronchus supplies the left lung.
- They further divide into secondary (lobar) bronchi inside the lungs.
4. What is the difference between the trachea and the bronchi?
The trachea is a single airway that carries air from the larynx, while the bronchi are two branches that conduct air into each lung.
- The trachea has C-shaped cartilage rings.
- Stem bronchi have cartilage plates instead of complete rings.
- The trachea lies outside the lungs, whereas bronchi enter and branch within the lungs.
5. What is the structure of the trachea?
The trachea is composed of multiple layers designed to keep the airway open and clean.
- Mucosa lined with ciliated pseudostratified columnar epithelium.
- Submucosa containing glands that secrete mucus.
- C-shaped hyaline cartilage rings for support.
- Trachealis muscle at the posterior side to adjust diameter.
6. Why are the cartilage rings important in the trachea?
The cartilage rings in the trachea prevent airway collapse during breathing.
- They maintain an open lumen for continuous airflow.
- Their C-shape allows the esophagus to expand during swallowing.
- They provide flexibility while maintaining rigidity.
7. What is the carina of the trachea?
The carina is the internal ridge at the lower end of the trachea where it divides into the right and left stem bronchi.
- It is located at the level of T4–T5 vertebrae.
- It is highly sensitive and triggers the cough reflex when irritated.
- It marks the beginning of bronchial branching.
8. Why is the right stem bronchus more prone to foreign body entry?
The right stem bronchus is more prone to foreign body entry because it is wider, shorter, and more vertical than the left bronchus.
- It forms a smaller angle with the trachea.
- Inhaled objects tend to follow the more direct pathway.
- This anatomical feature is clinically important in aspiration cases.
9. How does the trachea clean and filter the air we breathe?
The trachea cleans inhaled air using the mucociliary escalator mechanism.
- Mucus traps dust, microbes, and particles.
- Cilia beat upward toward the pharynx.
- Trapped particles are swallowed or expelled by coughing.
10. What happens to the stem bronchi inside the lungs?
Inside the lungs, the stem bronchi branch repeatedly to form a network called the bronchial tree.
- They divide into secondary (lobar) bronchi.
- Then into tertiary (segmental) bronchi.
- Further branching forms bronchioles that lead to alveoli for gas exchange.
