The respiratory portion consists of respiratory tract bronchioles, alveolar ducts, alveolar sacs and alveoli. That is hard to inspection the organisation of these structures in sections, because when the lungs space removed, castle collapse. Usually the respiratory system is composed of a branching collection of air spaces, which space in nearby proximity come pulmonary capillaries. The air room is exchanged roughly 10 to 15 time a minute. The air spaces room within 0.2µm the the blood, i beg your pardon is a really thin obstacle to diffusion. This arrangement means there is a fast reliable transfer of oxygen and also carbon dioxide between the blood and also the air, the major function that the respiratory tract portion.

The terminal bronchioles branch to provide rise to respiratory bronchioles, which lead to alveolar ducts, alveolar sacs and alveoli.

You are watching: The wall of the alveolus (air sac) in the lung is composed of which type of epithelium

This diagram shows a diagram of one alveolar sac, showing just how the organization of the alveoli, and also the network of blood capillaries that surround the alveoli (in red). These capillaries are derived from the pulmonary arterioles.

Gaseous exchange in between the blood and also air takes place in the alveoli, but the comprehensive structure that the alveolar wall surfaces cannot be fixed with the irradiate microscope.




The epithelium of the alveoli, contains two main types of cells:

1. Kind I pneumocytes: large flattened cells - (95% that the total alveolar area) which present a an extremely thin diffusion barrier for gases. They are linked to each other by tight junctions.

2. type II pneumocytes (making increase 5% the the complete alveolar area, but 60% that total variety of cells). These cells secrete "surfactant" i beg your pardon decreases the surface ar tension in between the slim alveolar walls, and stops alveoli collapsing when you breathe out. This cells are linked to the epithelium and also other constituent cell by chop junctions.

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The surfactant is comprised of phospholipids, combined with carbohydrate and protein, which are released by exocytosis, and type a tubular lattice of lipoprotein. The surfactant overcomes surface ar tension, wherein the two alveolar surface come together. Otherwise the two thin alveolar walls can stick together, rather like a balloon the is deflated, after gift inflated.

Macrophages are vital for eating bacteria and particles, and arise native monocytes, which have escaped native the blood capillaries.


This diagram shows the main constituents that alveolus, and also the interalveolar wall. The thickness the the alveolar-capillary barrier varies native 0.2 come 2.5 µm. The wall of the capillary endothelial cabinet is fused to that of the alveolar cabinet with only a really thin basement membrane between these two cells. This to produce a very small gap across which oxygen and also carbon dioxide have the right to rapidly diffuse.