From the respiratory bronchioles onwards, the structures contain alveoli in their walls and serve both as conducting and respiratory passages. The epithelium is initially simple cuboidal but become flattened or squamous later. No Goblet cells or glands occur in any part.
- The respiratory structures consist;
- Respiratory bronchiole,
- Alveolar duct,
- Alveolar Sac,
- Alveoli.
Respiratory bronchiole
- These bear alveoli in their walls and between the alveoli, the wall is lined by simple cuboidal epithelium, which may be ciliated in the initial portion.
- Smooth muscle and elastic fibres are present, though they do not form a thick layer as in terminal bronchiole.
- They are arranged in a spiral direction forming a loose elastic and contractile network.
- The respiratory bronchioles branch into alveolar ducts.
Alveolar ducts
- These are long and branch repeatedly. Their walls are formed by alveoli, without intervening patches of cuboidal epithelium.
- Smooth muscle bundles are present and are concentrated around the openings of alveoli.
- There is no separate epithelium other than that of the alveoli which consists of flattened or exceedingly thin squamous cells. The alveolar ducts open into a variable number of alveolar sacs.
Alveoli
- These are cup-shaped structures, through the wall of which interchange of gases between the blood and air takes place. An alveolus may open into the lumen of respiratory bronchiole, or alveolar duct or alveolar sac.
- Between adjacent alveoli, an inter alveolar septum is formed. The wall of an alveolus contains a network of fine elastic and reticular fibres and in the meshes of this network is a rich plexus of capillaries. The lumen of the alveolus is lined by simple squamous epithelium made up of very thin attenuated cells.
- The cells shows such a thin layer of cytoplasm that except at the region of the nucleus, no outlines can be made out in routine preparation under light microscope.
- EM studies have shown that this epithelium rests on a basement membrane wherever a capillary lies close to the alveolar wall, the basement membrane of the capillary endothelium and that of the alveolar epithelium fuse together. This arrangement results in a minimum of tissue intervening between air in the alveolus and blood in the capillary, thus facilitating gaseous exchange between the two.
- In the alveolar wall, in addition to the lining epithelial cells and endothelial cells of the capillary, connective tissue cells often referred to as septal cells are present. These correspond to macrophages and have the potency to become actively phagocytic in function.
Blood vessels
- The Capillary nets described above are derived from intralobular arteries which are branches of pulmonary artery (functional blood) and which go along with the terminal bronchiole.
- The interlobular arteries (nutrient arteries) are branches of bronchial artery and supply bronchial wall and interstitial tissue.
- The satellite veins are pulmonary and bronchial veins respectively.
