What are the alveolar wall and capillary wall

The alveolar wall and capillary wall are key structures for gas exchange in the lungs. The alveolar wall is composed of a single layer of flat epithelial cells, while the capillary wall is composed of endothelial cells and basement membrane, which together form the respiratory membrane. The alveolar wall is a thin membrane structure on the surface of the alveoli, mainly composed of type 1 alveolar cells and type 2 alveolar cells. Type I alveolar cells are flat and cover most of the alveolar surface, responsible for gas exchange. Type II alveolar cells are cubic in shape, with a small number but a large volume. They can secrete pulmonary surfactant, reduce alveolar surface tension, and prevent alveolar collapse. The alveolar wall also contains a small number of macrophages, responsible for clearing inhaled dust and microorganisms. The thickness of the alveolar wall is extremely thin, which facilitates the rapid diffusion of oxygen and carbon dioxide. The capillary wall is composed of a single layer of endothelial cells and their basement membrane, which are connected by tight junctions to form a continuous vascular lining. The capillary wall has selective permeability, allowing small molecules such as oxygen, carbon dioxide, and water to freely pass through, but restricting the passage of large molecules and blood cells. The capillary wall is closely adjacent to the alveolar wall, with only a very thin basement membrane separating them, which greatly shortens the distance of gas diffusion. Capillary endothelial cells can also secrete various bioactive substances, which participate in the regulation of vascular tone and inflammatory response. The alveolar wall and capillary wall together form the respiratory membrane, which is the structural basis for gas exchange in the lungs. The total thickness of the respiratory membrane is usually less than 1 micron, and this extremely thin structural feature ensures rapid diffusion of oxygen and carbon dioxide. In pathological conditions such as pulmonary edema or pulmonary fibrosis, thickening of the respiratory membrane can lead to gas exchange disorders. maintaining good respiratory function requires avoiding smoking, reducing exposure to air pollution, and regularly engaging in lung function exercises such as deep breathing and moderate aerobic exercise. When symptoms such as persistent cough and difficulty breathing occur, timely medical examination should be sought.