Introduction
The lung parenchyma consists of a large number of alveoli (air sacs) that are involved in gas exchange. The lung parenchyma occupies a large surface area in the chest cavity. The connective tissue fibers and the surfactant system maintain the mechanical stability of the parenchyma. Disturbances in the functioning of the parenchyma cause lung parenchymal diseases.
What Is Lung Parenchyma?
The lung is a complex organ with an internal structure that is vital for gas exchange. The lung parenchyma consists of a large number of alveoli and delicate fibers to support the intralobular alveolar portion. The lung parenchyma refers to the alveoli and any form of lung tissue which includes bronchioles, bronchi, blood vessels, and interstitium. It is also called the alveolar interstitium. The internal surface of the alveoli is lined by epithelium that is covered by a thin liquid film, and the extracellular matrix is composed of collagen and elastin fibers embedded in a proteoglycan matrix. The lung parenchyma is the vital portion of the lung that participates in gas exchange. The parenchymal region of the lung comprises about 90% of the total lung volume. The lung parenchyma forms a part of the conduction system, but it is mainly involved in gas exchange. The lung parenchyma, which consists of alveoli and supporting fibers, is divided into lobes and segments.
How Does Lung Parenchyma Develop?
The development of the lower respiratory tract begins on the 22nd day of gestation, and the formation of the trachea, lungs, bronchi, and alveoli continues. The bronchial buds formed during the fourth week divide asymmetrically to form two secondary bronchial buds on the left side and three buds on the right side. This gives rise to lobes of the lungs in the future. The secondary bronchial buds divide into tertiary bronchial buds on each side at the end of the sixth week. This later forms the bronchopulmonary segments of the mature lung. The alveolar ducts are formed between 16 to 25 weeks.
Lamellar bodies are formed during week 20. Lamellar bodies store the surfactant that is composed of lipids and surfactant proteins. The type II pneumocytes differentiate into squamous type I pneumocytes that later form the structural epithelium of alveoli. Some respiration is possible at this stage due to the formation of the gas exchange portion. The gas exchange portion of the lungs expands during 24 weeks of birth. The process of lung development begins early during fetal development, but complete maturation does not happen until the child is approximately eight years of age. Before birth, the immature alveoli are present as bulges from the sacculi that invade the primary septa. As the sacculi increase in size, they are divided into alveoli by the septa.
The alveoli divide until three years of age, with the majority of divisions occurring within the first six months. The increasing number of alveoli increases the size of the lungs until the third year of life. The size and number of the alveoli continue to increase until mature lungs are formed at around eight years of age.
What Is the Function of Lung Parenchyma?
The major function of the lung parenchyma is gas exchange. The transpulmonary pressure or prestress keeps the alveoli open. The tissue forces and alveolar surface film forces balance the transpulmonary pressure. The parenchymal architecture, prestress, and mechanical properties of the parenchyma are essential for efficient gas exchange. The alveolar surface film and the properties of parenchymal tissues are important components of lung tissue resistance. The prestress is essential for lung deformability. It influences local ventilation, regional blood flow, tissue stiffness, smooth muscle contractility, and alveolar stability. The biomechanical properties of the lung parenchyma play a role in maintaining normal lung function.
The mechanical properties of the parenchyma are determined by collagen, elastin, and proteoglycans. The mechanical behavior is led by the complex organization and the viscoelastic properties of the tissue components. Collagen in the parenchyma is the vital load-bearing component. A thin liquid film containing the pulmonary surfactant is present in the airways and alveoli. The macroscopic mechanical properties are determined by the surface tension. It plays a role in lung recoil and also influences the macrophysiology of the lungs by ensuring alveolar stability and also prevents collapse at low lung volume by reducing the surface tension. The biomechanical and structural properties, along with the prestress, determine the functioning of normal and diseased lungs.
What Is the Significance of Lung Parenchyma?
The lung parenchyma is composed of various proteins and profibrotic elements. These proteins result in the accumulation of connective tissue after repeated cycles of activation. This results in the thickening and scarring of the tissue and leads to fibrosis. This affects the alveolar interstitium, alveolar epithelium, pulmonary capillary endothelium, basement membrane, and perivascular and perilymphatic tissues. During instabilities, the body generates the right amount of tissue to repair the process, but the repair process is disrupted in parenchymal diseases and results in the thickening and scarring of the tissues and reduces the oxygen availability in the blood.
What Are Lung Parenchymal Diseases?
Mechanical dysfunction in the lung parenchyma, exposure to toxins (asbestos, coal dust, grain dust, silica, or talc), infections, or autoimmune diseases like rheumatoid arthritis can cause lung parenchymal diseases. Mechanical dysfunction leads to pulmonary fibrosis and emphysema. Other parenchymal lung diseases are interstitial pneumonia, idiopathic pulmonary fibrosis, nonspecific interstitial pneumonitis, hypersensitivity pneumonitis, cryptogenic organizing pneumonia, acute interstitial pneumonitis, desquamative interstitial pneumonitis,
Sarcoidosis, and asbestosis. Atelectasis or lung collapse is one form of lung parenchymal disease. Lung parenchymal diseases result in progressive scarring and fibrosis and disrupt oxygen availability.
What Are the Complications of Lung Parenchymal Diseases?
Lung parenchymal diseases can cause permanent damage to the lungs that cannot be cured. It can cause complications like
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Lung collapse.
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Lung infections.
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Respiratory failure.
Conclusion
The lung parenchyma is the vital portion of the lung that participates in gas exchange. It constitutes the major portion of the lungs and holds 90 % of the space in the chest cavity. The lung parenchymal mechanics, physiology, and cell biology are responsible for maintaining normal lung function and responding to lung injury or diseases.
