The trachea carries air into the chest, where it divides into branches of smaller diameter (bronchi) that deliver air to the lungs. Bifurcating, the bronchi form a system of gradually diminishing tubules reaching all parts of the lungs. They end with microscopic alveolar sacs, of which the lung tissue consists. It is in these thin-walled bubbles that gas exchange takes place between the inhaled air and blood. The upper and lower respiratory tract is the topic of the article.
Trachea
The trachea starts from the cricoid cartilage, located just below the larynx, and descends into the chest cavity. At the level of the sternum, the trachea ends, dividing into two branches - the right and left main bronchi. Trachea consists of a strong fibroelastic tissue with a chain of non-closed rings of hyaline cartilage (cartilage of the trachea). A trachea of an adult is enough (about 2.5 cm in diameter), while in infants on it is much smaller (about a pencil in diameter). The posterior part of the trachea does not have a cartilaginous support. It consists of fibrous tissue and muscle fibers. This part of the trachea lies to the esophagus located directly behind it. Trachea in cross section is an open ring. The epithelium (inner lining) of the trachea contains goblet cells that secrete mucus on its surface, as well as microscopic cilia, which, by coordinated movements, catch dust particles and push them away from the lungs to the larynx. Between the epithelium and the cartilaginous ring is a layer of connective tissue containing small blood and lymph vessels, nerves and glands that produce watery mucus in the lumen of the trachea. In the trachea, there are also a number of elastic fibers that give it flexibility. The main bronchus continues to branch, forming a so-called bronchial tree, carrying air to all parts of the lung. Primarily the main bronchus is divided into lobar bronchi, which is three in the right lung, and two in the left lung. Each of them delivers air to one of the lobes of the lung. The lobar bronchi are divided into smaller ones that provide air to separate channels.
Structure of the bronchi
The structure of the bronchi resembles the structure of the trachea. They are very soft and flexible, their walls contain cartilage, and the surface is lined with respiratory epithelium. They also have a variety of muscle fibers, which ensure a change in their diameter.
Bronchioli
Inside the bronchopulmonary segments, the bronchi continue to branch out. With each branching, the bronchi become narrower, with the total cross-sectional area increasing. Bronchi, having an internal diameter of less than 1 mm, are called bronchioles. From large bronchial tubes, bronchioles differ in that their walls do not contain cartilage and slime cells on the inner lining. However, as well as bronchi, they have muscle fibers. Further branching leads to the formation of terminal bronchioles, which, in turn, are divided into the smallest respiratory bronchioles. Breathing bronchioles are called so because they directly communicate with the lumen of some alveoli. However, they leave the bunches from the alveolar ducts, branching from respiratory bronchioles.
Alveoli
Alveoli are tiny empty sacs with extremely thin walls. Gas exchange occurs in them. It is through the walls of the alveoli that oxygen from the inhaled air enters the pulmonary circulation through diffusion, and the final product of respiration, carbon dioxide, is released to the outside with exhaled air. The human lungs contain hundreds of millions of alveoli, which together constitute a huge surface (about 140 m2), sufficient for gas exchange. Alveoli form clusters resembling bunches of grapes, located around the alveolar courses. Each alveolus has a narrow lumen that opens into the alveolar course. In addition, there are microscopic holes (pores) on the surface of each alveolus, through which it communicates with neighboring alveoli. Their walls are lined with a flat epithelium. The alveoli also contain two types of cells: macrophages (protective cells), foreign particles that enter the lungs through the respiratory tract, and cells that produce surfactant - an important biological component.