![\"\"](\"https:\/\/meddists.com\/learn\/wp-content\/uploads\/2022\/06\/lung_histology-1024x604.png\")
Structure<\/h4>\n<\/span>\n\n\nThe respiratory part of the lungs includes the respiratory bronchioles<\/strong>, <\/strong>which give rise to the alveolar ducts<\/strong>, which in turn give rise to the alveolar sacs<\/strong> that open up to a cluster of alveoli<\/strong>.<\/p>\n\n\n\n- Alveolar ducts<\/strong> resemble “hallways” bordered almost exclusively by alveoli, and surrounded by smooth-muscle cells and connective tissue.<\/li>
- Alveolar sacs<\/strong> are the distal ends of the alveolar ducts, opening up to clusters of alveoli.<\/li>
- Alveoli <\/strong>are air spaces surrounded alveolar cells and capillaries, which allow the exchange of gases between the blood and air.<\/li>
- An interalveolar septa<\/strong> separates one alveolus from its neighbor, with interalveolar pores<\/strong> (of Kohn<\/em>) allowing collateral air circulation<\/strong> between adjacent alveoli.<\/li><\/ul>\n\n\n<\/span>\n
Cells<\/h4>\n<\/span>\n\n\nThe alveolar epithelium consists of alveolar cells<\/strong>, with the occasional brush cells<\/strong>, alveolar macrophages<\/strong>, and fibroblasts<\/strong>.<\/p>\n\n\n\n- Alveolar cells<\/strong> (pneumocytes<\/strong>) are divided into two types of cells:
- Type I alveolar cells<\/strong> (type I pneumocytes) are thin, squamous cells that comprise ~40% of the alveolar lining cells, but form ~95% of the surface lining of the alveoli, thanks to their massive surface area.
- Type I alveolar cells are not <\/strong>capable of self-regeneration; instead, type II alveolar cells proliferate and differentiate into type I alveolar cells.<\/li><\/ul><\/li>
- Type II alveolar cells<\/strong> (type II pneumocytes) are the predominant cell type in the alveoli (~60% of cells); they are cuboidal cells that project into the alveolar air space, and function to produce and secrete alveolar surfactant<\/strong>, as well as serve as stem cells for both types of alveolar cells.
- Due to their shape, type II alveolar cells form only ~5% of the surface lining of the alveoli, even though they are more numerous.<\/li><\/ul><\/li><\/ul><\/li>
- Brush cells<\/strong> are receptor cells are columnar receptor cells with blunt microvilli, similar to those found in the rest of the respiratory tract. They are theorized to monitor the quality of air within the alveoli.<\/li>
- Alveolar macrophages<\/strong> are a special type of phagocytes found both in the connective tissue of the alveolar tree, and in the air space of the alveoli. They ingest and remove inhaled particles.
- Alveolar macrophages are derived from monocytes, and they are a part of the MPS (mononuclear phagocyte system).<\/li>
- Coniophages<\/strong> (or dust cells<\/em>) is the name given to alveolar macrophages once they ingest dust particles, and siderophages <\/strong>(or heart failure cells<\/em>) are alveolar macrophages that have ingested hemosiderin, a product of RBC breakdown (seen in the lungs during heart failure).<\/li><\/ul><\/li>
- Fibroblasts <\/strong>produce and secrete connective tissue fibers, most importantly elastic and reticular fibers.<\/li><\/ul>\n\n\n<\/span>\n
Surfactant<\/h4>\n<\/span>\n\n\nThe massive surface area of the alveoli also means they suffer from a large surface tension, which would forces them to collapse.<\/p>\n\n\n\n
This, however, does not occur thanks to a special substance known as surfactant<\/strong>, which reduces the surface tension of the alveoli. It is produced by type II alveolar cells, and covers the inner surface of the alveoli. <\/p>\n\n\n\nIt consists of phospholipids (with the most important one being dipalmitoylphosphatidylcholine<\/strong>, or DPPC <\/strong>for short, which is the key component in reducing surface tension), and 4 key proteins:<\/p>\n\n\n\n- Surfactant protein A (SP-A)<\/strong>. The most abundant surfactant protein; functions to regulate surfactant production and secretion, as well as immune activity, forming an allergic response<\/li>
- P-B.<\/strong> Helps spread the surfactant into a thin film that equally coats surface of the alveoli.<\/li>
- SP-C<\/strong>. Aids in spreading the surfactant, and maintaining the orientation of the DPPC. <\/li>
- SP-D<\/strong>. A major protein with immune function. Binds to micro-organisms and lymphocytes, participating in inflammatory response.<\/li><\/ol>\n\n\n<\/span>\n
Blood-air-barrier<\/h4>\n<\/span>\n\n\n\n\nThe blood-air-barrier marks the border between the inspired alveolar air and the blood. It is composed of 5 layers:<\/p>\n\n\n\n
- Surfactant<\/strong><\/li>
- Alveolar epithelium<\/strong>, consisting of type I and II alveolar cells (pneumocytes)<\/li>
- Basal lamina of the alveolar cells<\/strong><\/li>
- Basal lamina of the endothelial cells<\/strong><\/li>
- Endothelial cells<\/strong><\/li><\/ol>\n\n\n\n
The blood-air-barrier has a thin portion<\/strong> (where gaseous exchange occurs) and a thick portion<\/strong> in which connective tissue cells reside and fluids accumulate within the basal laminae as part of the normal maintenance of the alveolar system.<\/p>\n\n\n\nIn many cases, the basal laminae (of the alveolar and endothelial cells) are fused together.<\/p>\n<\/div>\n\n\n\n
- Alveolar ducts<\/strong> resemble “hallways” bordered almost exclusively by alveoli, and surrounded by smooth-muscle cells and connective tissue.<\/li>
- Alveolar sacs<\/strong> are the distal ends of the alveolar ducts, opening up to clusters of alveoli.<\/li>
- Alveoli <\/strong>are air spaces surrounded alveolar cells and capillaries, which allow the exchange of gases between the blood and air.<\/li>
- An interalveolar septa<\/strong> separates one alveolus from its neighbor, with interalveolar pores<\/strong> (of Kohn<\/em>) allowing collateral air circulation<\/strong> between adjacent alveoli.<\/li><\/ul>\n\n\n<\/span>\n
Cells<\/h4>\n<\/span>\n\n\n
The alveolar epithelium consists of alveolar cells<\/strong>, with the occasional brush cells<\/strong>, alveolar macrophages<\/strong>, and fibroblasts<\/strong>.<\/p>\n\n\n\n
- Alveolar cells<\/strong> (pneumocytes<\/strong>) are divided into two types of cells:
- Type I alveolar cells<\/strong> (type I pneumocytes) are thin, squamous cells that comprise ~40% of the alveolar lining cells, but form ~95% of the surface lining of the alveoli, thanks to their massive surface area.
- Type I alveolar cells are not <\/strong>capable of self-regeneration; instead, type II alveolar cells proliferate and differentiate into type I alveolar cells.<\/li><\/ul><\/li>
- Type II alveolar cells<\/strong> (type II pneumocytes) are the predominant cell type in the alveoli (~60% of cells); they are cuboidal cells that project into the alveolar air space, and function to produce and secrete alveolar surfactant<\/strong>, as well as serve as stem cells for both types of alveolar cells.
- Due to their shape, type II alveolar cells form only ~5% of the surface lining of the alveoli, even though they are more numerous.<\/li><\/ul><\/li><\/ul><\/li>
- Brush cells<\/strong> are receptor cells are columnar receptor cells with blunt microvilli, similar to those found in the rest of the respiratory tract. They are theorized to monitor the quality of air within the alveoli.<\/li>
- Alveolar macrophages<\/strong> are a special type of phagocytes found both in the connective tissue of the alveolar tree, and in the air space of the alveoli. They ingest and remove inhaled particles.
- Alveolar macrophages are derived from monocytes, and they are a part of the MPS (mononuclear phagocyte system).<\/li>
- Coniophages<\/strong> (or dust cells<\/em>) is the name given to alveolar macrophages once they ingest dust particles, and siderophages <\/strong>(or heart failure cells<\/em>) are alveolar macrophages that have ingested hemosiderin, a product of RBC breakdown (seen in the lungs during heart failure).<\/li><\/ul><\/li>
- Fibroblasts <\/strong>produce and secrete connective tissue fibers, most importantly elastic and reticular fibers.<\/li><\/ul>\n\n\n<\/span>\n
Surfactant<\/h4>\n<\/span>\n\n\n
The massive surface area of the alveoli also means they suffer from a large surface tension, which would forces them to collapse.<\/p>\n\n\n\n
This, however, does not occur thanks to a special substance known as surfactant<\/strong>, which reduces the surface tension of the alveoli. It is produced by type II alveolar cells, and covers the inner surface of the alveoli. <\/p>\n\n\n\n
It consists of phospholipids (with the most important one being dipalmitoylphosphatidylcholine<\/strong>, or DPPC <\/strong>for short, which is the key component in reducing surface tension), and 4 key proteins:<\/p>\n\n\n\n
- Surfactant protein A (SP-A)<\/strong>. The most abundant surfactant protein; functions to regulate surfactant production and secretion, as well as immune activity, forming an allergic response<\/li>
- P-B.<\/strong> Helps spread the surfactant into a thin film that equally coats surface of the alveoli.<\/li>
- SP-C<\/strong>. Aids in spreading the surfactant, and maintaining the orientation of the DPPC. <\/li>
- SP-D<\/strong>. A major protein with immune function. Binds to micro-organisms and lymphocytes, participating in inflammatory response.<\/li><\/ol>\n\n\n<\/span>\n
Blood-air-barrier<\/h4>\n<\/span>\n\n\n
\n\nThe blood-air-barrier marks the border between the inspired alveolar air and the blood. It is composed of 5 layers:<\/p>\n\n\n\n
- Surfactant<\/strong><\/li>
- Alveolar epithelium<\/strong>, consisting of type I and II alveolar cells (pneumocytes)<\/li>
- Basal lamina of the alveolar cells<\/strong><\/li>
- Basal lamina of the endothelial cells<\/strong><\/li>
- Endothelial cells<\/strong><\/li><\/ol>\n\n\n\n
The blood-air-barrier has a thin portion<\/strong> (where gaseous exchange occurs) and a thick portion<\/strong> in which connective tissue cells reside and fluids accumulate within the basal laminae as part of the normal maintenance of the alveolar system.<\/p>\n\n\n\n
In many cases, the basal laminae (of the alveolar and endothelial cells) are fused together.<\/p>\n<\/div>\n\n\n\n
- Alveolar epithelium<\/strong>, consisting of type I and II alveolar cells (pneumocytes)<\/li>
- P-B.<\/strong> Helps spread the surfactant into a thin film that equally coats surface of the alveoli.<\/li>
- Fibroblasts <\/strong>produce and secrete connective tissue fibers, most importantly elastic and reticular fibers.<\/li><\/ul>\n\n\n<\/span>\n
- Alveolar macrophages<\/strong> are a special type of phagocytes found both in the connective tissue of the alveolar tree, and in the air space of the alveoli. They ingest and remove inhaled particles.
- Type II alveolar cells<\/strong> (type II pneumocytes) are the predominant cell type in the alveoli (~60% of cells); they are cuboidal cells that project into the alveolar air space, and function to produce and secrete alveolar surfactant<\/strong>, as well as serve as stem cells for both types of alveolar cells.
- Type I alveolar cells are not <\/strong>capable of self-regeneration; instead, type II alveolar cells proliferate and differentiate into type I alveolar cells.<\/li><\/ul><\/li>
- Type I alveolar cells<\/strong> (type I pneumocytes) are thin, squamous cells that comprise ~40% of the alveolar lining cells, but form ~95% of the surface lining of the alveoli, thanks to their massive surface area.
- Alveolar sacs<\/strong> are the distal ends of the alveolar ducts, opening up to clusters of alveoli.<\/li>
![\"\"](\"https:\/\/meddists.com\/learn\/wp-content\/uploads\/2022\/06\/blood-air-barrier.png\")
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