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The lungs are a pair of primary respiration organs located in the thoracic cavity on either side of the mediastinum. These organs are covered by a thin, double-layered serous membrane called the pleura.[1] The respiratory system consists of 2 components—the conducting and respiratory portions. The conducting portion transports air from the external environment to the site of respiration, whereas the respiratory portion facilitates gas exchange and blood oxygenation. The conducting portion of the respiratory system includes the nose, nasopharynx, larynx, trachea, and a series of progressively narrowing bronchi and bronchioles, terminating at the terminal bronchiole.[2] The respiratory portion starts at the respiratory bronchiole, extends through the alveolar ducts and alveolar sacs, and culminates in the alveoli, where the primary gas exchange occurs. The branching pattern of these conducting passages looks like the branching of a tree and is hence called the tracheobronchial tree.[3] The right lung has 3 lobes, whereas the left lung has 2 lobes. Each lobe is aerated by a secondary (lobar) bronchus. The lobes are further divided into smaller pyramidal-shaped sections called the bronchopulmonary segments. There are 10 bronchopulmonary segments in each lung, with their apex directed towards the hilum, and each segment is aerated by a tertiary (segmental) bronchus.[4] The alveoli are the structural and functional units of the respiratory system. An adult human has approximately 300 million alveoli, providing a surface area of around 80 square meters for gas exchange.[5] The lungs are an essential component of pulmonary circulation. Deoxygenated blood from the right ventricle is pumped through the pulmonary arteries to the alveolar-capillary beds of the lung for gaseous exchange. Oxygenated blood from the capillaries of the lungs is returned to the left atrium by the 4 pulmonary veins (see Image. The Lungs).[6]

The lung pathologies are varied, showing significant differences in their presentation due to the organ's exposure to the external environment. Lung pathologies can be broadly classified into obstructive and restrictive lung diseases. Lung pathologies include bronchial diseases, infectious diseases, interstitial lung diseases, neoplasms, vascular diseases, and congenital abnormalities. The destruction of lung parenchymal tissue presents chronic obstructive pulmonary disease and emphysema due to chronic inflammation.[34] Morphologically, they lead to an increase in size and number of small fenestrae in alveolar walls, the breakdown of fibrovascular trabeculae, and the remodeling of acini, leading to airspace enlargement.[35] In certain obstructive diseases, such as bronchitis, hyperplasia of goblet cells occurs, whereas in bronchiectasis, the bronchi are markedly dilated. Restrictive disorders, on the other hand, are marked by fibrous deposits that restrict lung function. Interstitial restrictive lung diseases are characterized by inflammation or scarring of the lung tissue or filling of the air spaces with exudate and debris. Extrapulmonary restrictive diseases show a thickening of alveolar septa and epithelium and are associated with an endothelial injury. In the acute phase, restrictive lung disease demonstrates endothelial damage. In some disorders, they also show epithelial damage with fibrosis of the exudate and expansion of the interstitium through generalized fibrosis.  Chronic restrictive lung diseases are marked by diffuse interstitial changes that are more prominent than morphological changes. In advanced stages, many chronic restrictive lung diseases are characterized by interstitial fibrosis, leading to a classic honeycomb-like appearance of the lung (see Image. Histological Section of Pneumonia-Affected Lung).[36]