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Definition
| The respiratory system can be divided into two parts. The first part, the conducting portion functions as a pathway for ambient air. This portion also removes particulate matter from the air, controls air temperature, and contains receptors for olfaction. A specialized organ within the conducting system, the larynx, is also responsible for human speech. The second part, the respiratory portion, consists of the respiratory bronchioles, the alveolar ducts, and the alveoli. The alveoli, thin sacs (resembling a "cluster of grapes") , are the sites where external respiration takes place within the lung tissue. |
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| Organs within the respiratory system have a tubular structure and are arranged in specific layers. This lamellar relationship is not as well defined as that within the digestive system but generally consists of the four layers de¬scribed below. |
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| This layer faces the lumen and consists of the respiratory epithelium (mainly pseudostratified ciliated columnar epithelium), and some connective tissue, the lamina propria. The lamina propria can contain glands, generally of the mixed or mucoserous type. Within the respiratory epithelium five distinct cell types are present. These are: |
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| a) the ciliated columnar cell: |
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| This cell contains about 300 cilia on its apical surface. In Immobile cilia syndrome (Kartagener's syndrome), dynein, the molecular motor for ciliary action, is deficient and cilia are immobile. Particulate matter can enter the lungs of patients with this disease unimpeded. |
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| This cell secretes mucin glycoproteins and helps to moisten the epithelial lining. |
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| These cells are distinguishable because numerous microvilli reside on their apical surface.. Axons have been seen in relation to these cells; the cells may have sensory function. |
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| These are the progenitor cells of other cell types in the respiratory system. |
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| This cell, similar to a basal cell, has been shown, with electron microscopy, to contain dense core granules 100-300 nm in diameter. Small granule cells secrete polypeptide hormones as part of the diffuse neuroendocrine system. It is hypothesized that these cells may function to control serous and mucus secretion in the respiratory system. |
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| The submucosa consists of loose connective tissue which contains varying amounts of elastic and collagen fibers. Many blood vessels and nerves are also present to nourish the epithelium. This layer often combines with the lamina propria as the tubules become smaller. |
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| 3) a layer of cartilage and smooth muscle |
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| or fibrous connective tissue covering. |
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| The nasal cavity is divided into the external vestibule, directly behind the external opening or nares, and the internal nasal fossa. |
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| This region is lined with non-keratinized stratified squamous epithelium and is a transitional zone between the keratinized stratified squamous of the skin and the respiratory membrane. Underlying sebaceous and sweat glands make this membrane a "wet membrane". Long hairs, termed vibrissiae, are present in the external vestibule and remove large particles from the air. |
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The nasal fossae are two large chambers, within the skull, which are separated by the nasal septum. The lateral walls of each chamber are formed by the three nasal conchae. The superior nasal concha is lined with pseudostratified ciliated columnar epithelium, the olfactory epithelium, and contains three cell types. These types are the sustentacular cell [a columnar cell with many microvilli], the basal cell [small cells at the base of the epithelium which are capable of regenerating the other cell types], and the olfactory cell [bipolar neurons which possess non-motile cilia]. These cells can be replaced many times during the lifetime, [an exception in nervous tissue] The olfactory cell is responsi¬ble for the generation of a receptor potential when contact is made between the receprors on the surface opf the cells and the odiferous substances. Within the lamina propria, the olfactory glands of Bowman, keep the olfactory surface moist so that odiferous substances can easily dissolve in the liquid covering the epithelial membrane.
The middle and inferior conchae are lined with a normal respiratory epithelium. The mucus on the surface of the pseudo¬stratified columnar epithelium traps dust and the ciliary action moves these particles into the throat. Periodic engorgement of the swell bodies, venous plexuses present in the lamina propria of the concha, direct the flow of ambient air to either side of the nasal cavity at regular intervals, preventing the nasal epithelium on the opposite side from desiccation. Allergic reactions can cause permanent engorgement of the swell bodies. Ambient air is warmed by the extensive capillary network of this region as it twirls across the tubinate bones which form the framework of the conchae. |
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Definition
| These sinuses are found within the frontal, maxillary, sphenoid, and ethmoid bones. They consist of a respi¬ratory epithelium containing many goblet cells, a thin lamina propria, and a submucosa which attaches to the periosteum. Mucus produced in the sinuses drains into the nasal cavity through small openings which, if obstructed, can cause pressure within the sinuses, or sinus headaches. Infections of the maxillary sinus are particularly prevalent. |
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| Cilia, in the pseudostratified columnar epithelium of the nasopharynx, move the dust-laden mucus towards the mouth for expulsion. |
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| The larynx functions as a portion of the airway and as an organ of phonation. |
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| The mucosal lining of the larynx is loosely adherent to the cartilaginous framework except on the posterior surface of the epiglottis where the epithelium is extremely adherent and is thrown into two folds, the false vocal fold and the true vocal fold. The superior portion of the epiglottis and the true vocal fold, have a stratified squamous epithelial lining (maybe a little keratin) while the rest of the larynx is lined by pseudostratified ciliated epithel¬ium. |
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| Beneath the epithelium of the true vocal folds lie bundles of parallel elastic fibers , the vocal ligament. These bands are strung between the underlying cartilage like the strings on a guitar. Mucous and serous glands are present within the submucosa. |
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The cartilaginous framework consists of two sets of three paired and three unpaired cartilages. The three unpaired cartilages are large and include the thyroid cartilage (hyaline type), the cricoid cartilage (hyaline type) and the epiglottal cartilage (elastic type). The three paired cartilages are smaller and include the arytenoid cartilage (hyaline and elastic type) , the corniculate cartilage (elastic type), and the cuneiform cartilage (elastic type).
The larynx has both extrinsic muscles (muscles which origi¬nate elsewhere) and intrinsic muscles ( muscles originating within the larynx) which attach to it. The extrinsic muscles support the larynx and connect it with surrounding muscles and ligaments. The intrinsic muscles are numerous and are of the skeletal type. In the region of the true vocal cord, the vocalis muscle (part of the thyro-arytenoid muscle) underlies the vocal ligament and functions to regulate the tension of the folds and their ligaments, thus, producing various pitches of sound. |
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| The trachea is about 10 cm long and functions as a patent opening for the passage of air. For this purpose, it has a strongly supported muscle and cartilage framework. |
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| The mucosa is of the pseudostratified ciliated columnar type which rests on a thick basal lamina. A thick band of elas¬tic fibers separates the lamina propria from the submucosa. |
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This region contains a plexus of arteries, veins, nerves, and mucoserous glands.
Muscle, Cartilage, and Adventitia: The 15-20 C-shaped rings of hyaline cartilage in the larynx are open on their posterior end . Smooth muscle fibers, collectively termed the trachealis muscle, join the two ends and give flexibility to this strongly supported portion of the airway. The fibrous connective tissue covering over the trachea binds firmly to the cartilage and muscle. The C-shaped cartilages, muscle, and fibrous connective tissue have been sometimes referred to as the adventitial layer. |
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| The bronchial tree is a complex system of branch¬ing tubes. The amounts of cartilage and glandular material decline as the bronchi get smaller, however, elastic fiber concentration increases as the diameter of the tubule decreases. Elastic fibers, within the actual lung tissue, are responsible for the elasticity or atalectisis of the lung. The layered arrangement of the tubules becomes less distinct as the tubules become smaller. |
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| The trachea divides into two primary bronchi which further subdivide into lobar bronchi (3 in right lungand 2 in left lung), and then into segmental bronchi. |
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Definition
| Lined with pseudostratified ciliated columnar epithelium with a prominent basal lamina. Goblet cells are numerous. There are many elastic fibers in the lamina propria along with a criss¬crossing bundle of spirally arranged smooth muscle. Contraction of this muscle after death gives the epithelium the convoluted appearance distinguishable on histological sections. Lymph nodules can also be found and are numerous within the bronchial tree. |
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| Mixed glands appear in the submucosa between the muscle and cartilage and extend through the bronchial tree as far down as the cartilage. |
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| Hyaline cartilage is present appearing as complete rings within the primary bronchi and as plates of hyaline carti¬lage within the intrapulmonary bronchi (within the lungs). |
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Definition
| Fibrous connective tissue lining is continuous with that of the esophagus. |
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| These are very small intralobular (between the lobes of the lungs) tubes which have a diameter of 5mm or less. The larger bronchioles are lined with pseudostratified columnar epi¬thelium, however, the epithelium becomes more cuboidal in the smaller bronchioles. Goblet cells are greatly decreased in number within upper bronchioles and absent in the terminal bronchiole. Glands are absent. |
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| The Clara cell is present within the epithelium of the terminal bronchiole, and contains apical secretory granules known to be glycosaminoglycans which protect the bronchiolar lining. The lamina propria contains smooth muscle and elastic fibers. |
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Definition
| comprised of groups of 80-100 cells containing secretory granules which receive cholinergic nerve endings. These bodies are hypothesized to be chemoreceptors sensitive to changes in gas composition within the airway. Stimulation by the sympathetic system signals physical stress (fright, fight, and flight) causing bronchial dilation whereas stimulation by the vagus (parasympathetic) causes con¬striction of the airway at rest. Bronchioles have a relatively large amount of smooth muscle, and are the portion of the airway which undergoes spasm during an asthma attack. A definite submu¬cosa is absent as is the layer of cartilage. |
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| These bronchioles are less than 0.5 µm in diameter and lined with ciliated cuboidal epithelium in the upper portion and simple squamous epithelium in the lower portion at the point where many alveoli branch off from the alveolar wall. Clara cells are present in the upper bronchioles but disappear in the distal portions of the respiratory bronchiole. Smooth muscle and elastic fibers lie between the epithelial lining and the lumen of the bronchioles. |
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Definition
| Numerous alveoli branch from the wall of the alveolar duct. The epithelial lining at this point is a very thin layer of simple squamous epithelium. It was determined that the alveolar epithelium was an intact layer only after the devel¬opment of electron microscopy. Knobs of smooth muscle lie in the lamina propria below the epithelial lining and these knobs disap¬pear at the level of the atrium, a widening passageway which connects the alveolus with the alveolar duct. Elastic and colla¬gen fibers lie in the lamina propria. |
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Definition
| These structures are sac-like evaginations (200 µm in diameter) which represent the physiological unit of the respira¬tory system, the region in which external respiration takes place. Each alveolus is lined by a continuous layer of simple squamous epithelium comprised of Type 1 pneumocytes. A rich capillary network, collagen fibers, elastic fibers, fibroblasts, and macrophages are located in the interstitium between adjacent alveoli. Destruction of the interalveolar septum is a distinguishing sign of emphysema which results in large cyst-like alveoli in which the elastic framework has been extensively damaged. Emphysema is very common in older people and is directly linked to extensive air pollution and smoking. |
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| Alveoli: Blood-air barrier: |
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Definition
| The general function of the lungs concerns the process of external respiration, or the exchange of gases. To facilitate this process, the barrier between alveolar lumen and the lumen of the capillary is quite thin. This barrier is comprised of the Type 1 Pneumocyte [a simple squamous epithelial cell], a single basal lamina created by the fusion of the lamina of the alveolar epithelial and capillary endothelial cell, and the simple squamous endothelial cell of the capillary. Partial pressures created by differences in gas concentration on either side of this membrane cause oxygen to diffuse through the alveolar wall, dissolve in the blood plasma, and bind with the hemoglobin molecule of the RBC. Carbonic anhydrase, an enzyme present in the RBC, disassociates carbon dioxide from carbonic acid, freeing it to diffuse into the alveolar wall where it is excreted during expiration. |
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| Cell Types of the Alveolus: |
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| forms 95% of the alveolar surface and is of the simple squamous epithelial cell type. Organelles are clustered around the nucleus in this cell, leaving attenuated lateral surfaces. Pinocytotic vesicles are also present. Debris from the alveolar lumen is collected by the pinocytotic activity of Type 1 Pneumocytes and transferred to macrophages of the interstitium. |
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| The Type 2 Pneumocyte (Greater Alveolar or septal cells) |
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| a cuboidal cell with short microvilli on its apical surface. Electron microscopy shows that it is a typical secretory cell that contains lamellar bodies which give rise to pulmonary surfactant. Surfactant is in the form of a phospolipid film which has a high concentration of dipalmitoyl lecithin. Surfactant functions to decrease the surface tension on the alveolar cell surface. This decreases the amount of work re¬quired to inflate the alveoli and prevents the alveoli from deflating during expiration. Hyaline membrane disease in prema¬ture infants is caused by insufficient amounts of surfactant. |
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| (Dust Cell) originates within a cell population derived from monocytes. These cells are usually found within the interalveolar septum or lumen. Those in the lumen are called dust cells and may be found adhering to epithelial cells. Macrophages within the lung tissue function to defend the body against infection. They have surface receptors to IgG and the Cb3 component of complemen. and they function in ingesting bacteria.. In the presence of bacteria, dust cells produce chemoattractant factors which allow for neutrophil migration through the walls of adjacent blood vessels.. In smokers, the macrophages can become filled with undigested residues.. Macrophages in the alveoli can only be removed by expectoration and the rate of clearance has been estimated to be 2,000,000 macrophages /hour. |
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| bind to the basal surface of the alveolar epithelium. These cells react with antiactin and antimyosin antibodies and are considered to decrease the size of the alveolar lumen. |
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| Fibroblasts and Mast cells |
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| The function of lung tissue is to oxygenate blood, containing high levels of carbon dioxide, which has been returned to the right ventricle of the heart. The pulmonary circulation is comprised of pulmonary arteries (carry deoxygenated blood to the lungs) and pulmonary veins (return oxygenated blood back to the heart from the lungs). The pulmonary vessels do not supply nutrients to the lung cells. This function is provided by the bronchial vessels which arise from the internal thoracic artery and the thoracic aorta. Actual blood supply to the lung tissue is provided by the bronchial arteries, which are much smaller than the pulmonary vessels. An additional extensive lymphatic network surrounds the lungs and the pleura although lymphatics do not extend below the level of the alveolar ducts. |
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