Alveolar septum
The alveolar septum separates adjacent alveoli in lung tissue. The minimal components of an alveolar septum consist of the basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...

s of alveolar-lining epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 (mostly type I pneumocyte
Type I pneumocyte
Type I pneumocytes are responsible for gas exchange in the alveoli and cover a majority of the alveolar surface area . While type I pneumocytes account for most of the surface area, they are approximately half as numerous as type II cells, due to differences in size...

s) and capillary endothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...

. Thicker alveolar septa may also contain elastic fibers, collagen, interstitial cells, smooth muscle cells, mast cells, lymphocytes and monocytes.
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