Respiration is a key function of a living organism. Through the respiratory process, excess carbon dioxide moves out of the body, and oxygen necessary for the metabolic process is supplied to the different parts of the body. The key component of the respiratory system is the lungs. Different parts of the lungs fulfill different functions like carrying gas and exchange of gaseous substances. The anatomical and structural complexities of the lungs help in these processes.
What Is the Structure of the Lungs?
The lung is situated in the thoracic cavity. It is a conical structure with a broad base. The upper pointed portion of the lungs is known as the apex. Both right and left lungs are anatomically similar but structurally different. The right upper lobe, right middle lobe, and right lower lobe are the three different lobes present in the right lungs. The upper and lower lobes are two different lobes that are present in the left lungs.
The right lung is again still divided into ten segments. Three segments are present in the upper lobe, two are present in the middle lobe and five segments are present in the lower lobe. The left lung has nine segments. Five segments are present in the upper lobe and four segments are present in the lower lobe. The covering of the lungs is known as the pleura. Hilum is the portion of the lung through which the trachea enters the lungs. The portion of the trachea that enters the lungs is known as the bronchi. This portion is again subdivided into several other portions which serves as a functional unit of the lungs.
What Is Pleura?
The covering of the lungs is known as the pleura. It is a double-layer serous membrane that folds back and forms a sac. This sack is known as the pleural sac. The inner layer of the pleura is known as the visceral pleura. This layer covers the internal portion of the lungs, nerves, and blood vessels. The outer pleural layer is known as the parietal pleura. This layer again can be divided into three parts. These are:
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Mediastinal Part: Covers the mediastinum (the portion of the chest occupied by the heart) portion.
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Costal Part: Covers the inner portion of the thoracic cavity and ribs.
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Diaphragmatic Part: This portion covers the diaphragm (a curtain-like structure that separates the thorax and abdomen).
The space between the two layers is filled with a fluid-like substance. The serous fluid lubricates the space between two layers and reduces friction during breathing. The total amount of fluid in the pleural space is 2 to 10 millimeters. The combined thickness of two layers and pleural space is 0.5 millimeters. The function of the parietal pleura is the formation and removal of the pleura. Also, large particles are excreted out of the pleural cavity through the parietal pleura via stroma-like space with the help of the lymphatic system.
What Is Bronchi?
The function of the trachea or windpipe is to carry air. The part of the trachea that enters the lungs is known as the bronchi. At the level of the fourth thoracic vertebrae or sternal angle (portion where the second rib joins with the sternum), the trachea is divided into two parts and forms the bronchi. Each bronchus enters the respective side of the lungs and is known as the primary bronchi. The primary bronchi enter the lungs in the inferior and lateral direction through an area called hila. The right primary bronchus is wider and shorter, whereas the left primary bronchus is smaller in diameter. The primary bronchus is again subdivided into secondary lobar bronchi. On the right side, it is divided into three parts, and on the left side two parts. These secondary lobar bronchi are again subdivided into several tertiary segmental bronchi. On the right side, there are ten segmental bronchi supplying each bronchopulmonary segment, and on the right, there are eight segmental bronchi are present.
The structure of the bronchi and the subsequent segments are similar to the trachea. These segments are composed of cartilaginous rings, and smooth muscles, and are lined by pseudostratified columnar epithelium cells. In the main bronchi, the cartilages are completely ring-shaped whereas, in the segmental bronchi, the cartilages are crescent-shaped.
The segmental bronchi are further divided into smaller bronchioles. These bronchioles are considered the starting point of the gaseous exchange zone. These bronchioles are composed of elastic fibers and simple ciliated cuboidal epithelium. These bronchioles are again divided into 50 and 80 terminal bronchioles. Terminal bronchioles are composed of simple ciliated cuboidal epithelium and club cells. These cells are responsible for the secretion of surfactants. Terminal bronchioles give rise to respiratory bronchioles. These are the narrowest part of the respiratory ducts with a diameter of 0.5 millimeters. These respiratory bronchioles give rise to the six to eleven alveolar ducts and each duct is composed of five or six alveolar sacs. The oxygen-exchanging unit of the lungs is known as the alveoli. These are balloon-shaped portions that help in exchanging gases.
The alveoli occupy 90 percent of the lungs. The cluster of the alveoli is arranged to form a functional unit known as acini. The area between two alveoli is separated by the interalveolar septum. The design of inter alveolar septum makes it ideal for the exchange of gases. The tissue barrier present with the septa separates the air and blood. This tissue barrier consists of two cell layers. The epithelial cell layer faces the alveolar side and the endothelium faces the blood capillary side. The epithelial cell layer is again composed of two types of cells; known as type I and type II cell.
How Is the Lung Developed?
The development of the lungs can be divided into three parts the embryonic part, the fetal part, and the neonatal part. During the third week of pregnancy, the first respiratory diverticulum or lung buds appear from the primitive foregut. During the fifth week of the pregnancy, secondary bronchial buds appear from the primary bronchial buds. By the sixteenth week of gestation terminal bronchioles are formed. Till the twenty-sixth week, respiratory bronchioles and alveolar ducts are formed. This phase is known as the embryonic phase.
From the twenty-sixth week of gestation to birth the developmental phase of the lungs is known as the terminal sac period, during this period primitive alveoli are formed, and capillaries establish close contact. The final phase of lung development is the alveolar period. During this period the growth and maturation of the lungs occur. Also, epithelial cell growth around the mature alveoli takes place.
Conclusion
The main function of the lung is to help in breathing. It also helps in the gaseous exchange process. Secretion from various cells of the lungs provides protection from foreign substances. The alveoli of the lung is considered the functional unit of the respiratory process that helps in the breathing process.
