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.
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.
[Image will be Uploaded Soon]
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 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.