Vol. 9, Issue 12, Part A (2025)

Mammalian lungs (pulmones), a paired organ that occupies the majority of the thoracic cavity, are not identical in shape or size, with the right being significantly larger than the left. The lung is very elastic, flexible, and spongy. It floats in water and crepitates when pressed between the thumb and finger. The lung's elasticity and strain from external air pressure cause it to instantly collapse to around one-third of its original size and lose its normal structure when the thoracic cavity is opened. The anatomy of the lung is covered, with a particular emphasis on lung parenchyma. The lung parenchyma mainly consists of multiple air-containing channels and intervening fine structures, such as alveolar ductal lumens and alveoli, as well as alveolar septa and small pulmonary arteries, which account for 10% of total parenchymal volume. The structural morphology and three-dimensional architecture of the alveolar ducts and alveoli are closely explored, with a focus on their functional organization within the terminal respiratory unit. This is followed by a succinct description of the bronchial circulation and pulmonary lymphatic system, both of which play critical roles in tissue perfusion, fluid balance, and lung immune defense. The pulmonary surfactant is a lipoproteinaceous secretory complex made up of lipids and particular surfactant-associated proteins that the lungs' alveolar type II (AT-II) pneumocytes produce and secrete. It is a highly surface-active substance that creates a thin layer at the alveolar air-liquid interface, reducing surface tension and maintaining alveolar stability while preventing collapse during respiration. Throughout the respiratory cycle, pulmonary surfactant is essential for preventing alveolar collapse and shielding the lungs from infections and mechanical damage brought on by inhaled pathogens and foreign particles. It is essential for maintaining the structural integrity and functional stability of the alveolar architecture as well as for the effective diffusion of breathing gases.