Akademik Veri Yönetim Sistemi
A Histological Approach to Human Body Barriers, NOVA Publications , ss.207-220, 2025
The blood-air barrier, also called the alveolar-capillary barrier, is crucial for lung function as it allows the exchange of carbon dioxide (CO2) and oxygen (O2) between the pulmonary circulation and the alveolar air spaces. The alveolar epithelium, the extracellular matrix, and the pulmonary microvascular endothelium together form this barrier. Structurally, it consists of the cellular extensions of type I alveolar epithelial cells (AEC1s), the cellular extensions of the continuous endothelial cell layer, the common basal lamina of AEC1s and endothelial cells, and the plasma membrane of erythrocytes. The AEC1s cover about 95% of the alveolar surface and are remarkably thin to allow efficient gas exchange. The thickness of the alveolar-capillary barrier ranges from 0.1 to 1.5 μm and allows optimal gas diffusion while maintaining structural integrity. In addition, tight junctions between epithelial cells prevent leakage of fluid into the alveolar space, further supporting efficient gas exchange. Type II alveolar epithelial cells (AEC2s) secrete surfactant, a lipid-protein complex that, although not a structural component of the blood-air barrier, coats the alveolar surface, reduces surface tension, and averts alveolar atelectasis during exhalation. During the development of the respiratory system, the blood-air barrier undergoes significant changes in the shape and structure of the tissue. Although most mammals, including humans, initiate air respiration at birth, the lungs achieve functional maturity before birth. The formation of the blood-air barrier is an intricate process involving the differentiation and proliferation of alveolar epithelial cells, the establishment of the capillary network, and the deposition of extracellular matrix components. Proper formation and maintenance of this barrier is critical for normal respiratory function and overall health. Damage to the alveolar-capillary barrier increases lung permeability, disrupts gas exchange and leads to dyspnoea. Such damage can have various causes, including infection, inflammation and mechanical injury.