Respiratory system
Consists of the paired lungs and a series of air passages that lead to and from the lungs. As the air passages continue within the lung , they branch into increasingly smaller tubes until the very smallest air spaces, called alveoli ,are reached .
The passages external to the lungs consist of : Nasal cavities ( and , during forced breathing , the mouth ) , Nasopharynx and oropharynx , Larynx , Trachea and Paired primary bronchi .
Within the lungs are : Internal bronchi, which undergo extensive branching to give rise the bronchioles , then respiratory bronchioles , alveolar ducts , alveolar sacs, and alveoli .
Trachea
Is a short tube that extend from the larynx to about the middle of the thorax , where it divides into the two primary bronchi.
The wall of the trachea consists of four definable layers :
Mucosa , composed of ciliated , pseudostratified epithelium (contain mucous cells ) and an elastic fiber-rich lamina propria.
Submucosa, composed of a slightly more dense connective tissue than the lamina propria with atubulo -acinar glands ( tracheal gland ) .
Cartilaginous layer, composed of C-shaped hyaline cartilages ,between the two ends of each cartilage rings there is a smooth muscle called trachealis muscle.
Adventitia , which bind the trachea to adjacent structures .
1- Bronchi
Trachea divides into two branches right and left bronchi, they have the same general histological structure as the trachea ,but there is a small differences:
-Smaller in size.
-The cartilage rings are replaced by cartilage plates of irregular shape .
2- Bronchioles , are air-conducting ducts , lined by ciliated simple columnar to simple cuboidal epithelium , the lamina propria possesses no glands and is surrounded by a smooth muscle , supported by cartilage , they are branch repeatedly ,giving rise to smaller Terminal bronchiole .
3-Terminal bronchiole , that is lined by simple cuboidal epithelium , with reduced connective tissue, it also branched, they finally give rise to the Respiratory bronchiole.
4-Respiratory bronchiole, resemble terminal bronchioles but they have outpocketings of alveoli in their walls.
5-Alveolar ducts , are long straight tubes lined by simple squamous epithelium and display numerous alveoli ,they end in alveolar sacs.
6-Alveolar sacs , are composed of groups of alveoli clustered around a common air space .
7-Alveolus , is a small air space partially surrounded by highly attenuated epithelium, two types of cells are present in the lining , the first type pneumocytes (lining cells) and the second type dust cells pneumocytes (produce surfactant).