Occurring only in the respiratory portion (which their presence distinguishes from the conducting portion), these small (about 200 um in diameter) sacs open into a respiratory bronchiole, an alveolar duct, an atrium, or an alveolar sac. They are separated by thin walls termed interalveolar (or alveolar) septa.
A. Interalveolar Septa: The structural features of these septa, which are specialized for gas exchange, are critical to respiratory function. The septa consist of 2 simple squamous epithelial layers with the interstitium sandwiched between them. The interstitium consists of continuous (nonfenestrated) capillaries embedded in an elastic connective tissue that includes elastic and collagen fibers, ground substance, fibroblasts, mast cells, macrophages, leukocytes, and contractile interstitial cells that contract in response to epinephrine and histamine. This elastic tissue is an important component of the ventilating mechanism. Gas exchange occurs between the air in the alveolar lumen and the blood in the interstitial capillaries.
1. Blood-air barrier. This term refers to the structures that oxygen and CO2, must cross to be exchanged. Varying from 0. 1-1.5 um in thickness, it includes the following layers: a. The film of pulmonary surfactant on the alveolar surface. b. The cytoplasm of the squamous cpithelial (type I alveolar) cells. c. The fused basal laminae sandwiched between the type I alveolar and capillary endothelial cells. d. The cytoplasm of the squamous endothelial cells lining the intcrstitial capillaries.
2. Alveolar pores. Each septum may be interrupted by one or more pores from 10 to 15 um in diameter. These connect adjacent alveoli and may help to equalize pressure and allow collateral air circulation, thus maximizing the use of available alveoli when some small airways are blocked.
ALVEOLI
BRONCHIAL TREE - 2
D. Bronchioles: These are branches of the smallest bronchi. The largest bronchioles differ from the smallest bronchi only by the absence of cartilage and glands in their walls. Large bronchioles are lined by typical respiratory epithelium; as they branch further, the epithelial height and complexity decrease to simple ciliated columnar or cuboidal. Each bronchiole gives rise to 5-7 terminal bronchioles.
E. Terminal Bronchioles: The smallest components of the conducting portion of the respiratory system, these are lined by ciliated cuboidal or columnar epithelium and have few or no goblet cells. The lining here also includes dome-shaped cilia-free Clara cells, whose cytoplasm contains glycogen granules, lateral and apical Golgi complexes, elongated mitochondria, and a few secretory granules. The function of these cells is unclear. Each terminal bronchiole branches to form 2 or more respira tory bronchioles.
F. Respiratory Bronchioles: These are the first part of the respiratory portion, with a cuboidal epithelial lining which resembles that of the terminal bronchioles but which is interrupted by thin-walled saccular evaginations called alveoli. The number of alveoli increases as the respiratory bronchioles proceed distally. As the alveoli increase in number, the cilia decrease until they disappear. Goblet cells are absent.
G. Alveolar Ducts: These are simply the distal extensions of the respiratory bronchioles where the alveoli are so dense that the wall consists almost entirely of these sacs, and the lining has been reduced to small knobs of smooth muscle covered by cilia-free simple cuboidal cells.
H. Atria and Alveolar Sacs: Atria are the distal terminations of alveolar ducts. The arrangement is comparable to a long hallway (alveolar duct) leading to a rounded foyer (atrium). The foyer has small doorways leading to some small rooms (alveoli), but also has 2 or more larger doorways leading into short, dead-end hallways (alveolar sacs). The short hallways are also lined by small rooms (alveoli). Put simply, the difference between atria and alveolar sacs is that the atria open into alveolar ducts, alveoli, and alveolar sacs, while the alveolar sacs open only into alveoli and atria.
BRONCHIAL TREE
This begins where the trachea branches to form 2 primary bronchi, one of which penetrates the hilum of each lung. The hilum is also the site at which arteries and nerves enter and veins and lymphatic vessels exit the organ. These structures, together with the dense connective tissue that binds them, form the pulmonary root. The bronchial tree undergoes extensive branching within the lungs. The changes in wall structure that accompany the progress of the bronchial tree toward the alveoli occur gradually and not at sharp boundaries.
A. Primary Bronchi: There are 2 primary bronchi, one entering each lung. Their histologic appearance is quite similar to that of the trachea, but their cartilage rings and spiral bands of smooth muscle completely encircle their respective lumens. The path of the right primary bronchus is more vertical than that of the left. As a result, foreign objects that reach the bronchi are more likely to lodge in the right side of the bronchial tree.
B. Secondary Bronchi: These lobar broncbi are branches that arise directly from the primary bronchi; each supplies one pulmonary lobe. Since the right lung has 3 lobes and the left only 2, the right primary bronchus gives rise to 3 secondary bronchi and the left primary bronchus gives rise to 2. Their histologic structure is similar to that of the primary bronchi except that their supporting cartilages (and those of the smaller bronchi) are arranged as irregular plates, or islands, of cartilage, rather than as rings.
C. Tertiary Bronchi: Arising directly from the secondary bronchi, which they resemble histo logically, each of these segmental bronchi supplies one bronchopulmonary segment (pulmonary lobule). Although each lung has 10 such segments, the different number of secondary bronchi causes the tertiary branching pattern to differ between the right and left lungs. Except for a decrease in overall diameter, the histologic appearance of tertiary bronchi is identical to that of secondary bronchi. Tertiary bronchi may branch several times to form successively smaller branches, which are considered bronchi as long as their walls contain cartilage and glands.
TRACHEA
This 10-cm tube extends from the larynx to the primary bronchi. It is lined by respiratory epithelium, and its lamina propria contains mixed seromucous glands that open onto its lumen. Its most characteristic feature is the presence of 16-20 C-shaped cartilage rings whose open ends are directed posteriorly. The opening is bridged by a fibroelastic ligament that prevents overdistension as well as by smooth muscle bundles (tracbealis muscle) that constrict the lumen and increase the force of air flow during coughing and forced expiration.
Respiratory System Picture
Respiration occurs in following way from external surface to inner surface: Nasal cavity -pharnyx- trachea - bronch - bronchioles - arteries
The Details of these systems coming.
GENERAL FEATURES OF THE RESPIRATORY SYSTEM
A. Components and Basic Functions of the Respiratory System: The respiratory system includes the lungs, airways tie, pharnyx, larynx, trachea, bronchi) and associated structures. Specialized for gaseous exchange between blood and air, including the uptake of oxygen and release of carbon dioxide, it is functionally divisible into 2 major parts: the conducting and respiratory portions.
1. Conducting portion. The walls of this system of tubes are specialized to carry air to and from the site of gas exchange without collapsing under the pressures created by the ventilating mechanism. This portion also conditions the air, warming, moistening, and cleaning it to enhance gas exchange. It includes the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.
2. Respiratory portion. This portion is distinguished by alveoli, small, saccular structures whose thin walls enable the gas exchange between air and blood. Alveoli occur in clusters at the end of the bronchial tree. These clusters extend (like rooms from a hallway) from the walls of respiratory bronchioles, alveolar ducts, and atria and alveolar sacs.
B. Wall Structure: Like the digestive tract, the tubelike respiratory tract has layered walls whose lining epithelium derives from endoderm. The wall layers include an epithelium, a lamina propria that contains mucous glands as well as cartilage that prevent the tract from collapsing under pressure, smooth muscle that regulates the luminal diameter, and an adventitia that contains collagen and elastic fibers. Respiratory epithelium
a. General features. The epithelium lining most of the tract is ciliated pseudostratified columnar with goblet cells; it is generally referred to as respiratory epithelium. As the respiratory tract undergoes branching and its luminal diameter decreases, the epithelium gradually drops in height and loses first goblet cells and then cilia as it approaches the alveoli.
b. Epithelial cell types :
(1) Ciliated columnar cells predominate in the tract. Each has about 300 motile cilia on its apical surface; there are associated basal bodies in the apical cytoplasm.
(2) Mucous goblet cells are the second most numerous type. They secrete the mucus that covers the epithelium and traps and removes bacteria and other particles from inspired air. Cilia projecting from columnar cells sweep the contaminated mucus toward the mouth for disposal. (3) Brush cells. Also columnar, these cells lack cilia; they often have abundant apical microvilli. Two types are present: One resembles an immature cell and apparently serves to replace dead ciliated or goblet cells; the other has nerve endings on its basal surface and appears to be a sensory receptor.
(4) Basal cells. These small round cells lie on the basal lamina but do not reach the lumen. They appear to be stem cells that can replace the other cell types.
c. Metaplasia refers to the change in tissue organization or type undergone by epithelia in response to changes in the physical or chemical environment. For example, a smoker's respiratory epithelium typically develops more goblet cells in response to high pollutant levels and fewer ciliated cells in response to carbon monoxide. These changes, which are reversible, frequently cause congestion of the smaller airways.