RESPIRATORY SYSTEM

The respiratory system is formed of 2 functional components: –Conducting portion: for the transport of inspired and expired air between the atmosphere and the circulatory system. –Respiratory portion: for the exchange of gases between the atmospheric air and blood. Conducting parts include: –Nasal cavity –Pharynx –Larynx –Trachea –Bronchi (1ry & 2ry) –Bronchioles (terminal bronchioles) Respiratory parts: include: –Respiratory bronchioles –Alveolar ducts –Alveolar sacs (alveoli)

Extrapulmonary conducting portion –This extends from nasal cavity to primary bronchi. It is characterized by: Ciliated pseudostratified columnar epithelium rich in goblet cells. Submucous loose C.T. rich in mixed seromucous glands. Function of the conducting passages: –Trapping the inhaled particles and debris by the sero- mucous secretion which have suitable consistency to be expelled out towards the nose. Individuals who suffer from immobile cilia have chronic lung infections. The function of cilia is also lost in smokers thus inviting infections. –The seromucous secretion has detoxifying action on the soluble gases. –The subepithelial C.T. (lamina propria) contains diffuse lymphocytes, which produce secretory immunoglobulin A, which kills the bacteria and viruses and prevents them from penetrating the epithelium.

Anatomically the respiratory system is divided into 2 parts: –Upper respiratory tract. –Lower respiratory tract. The upper respiratory tract includes: –Nasal cavity. –Paranasal sinuses. –Nasopharynx. Its basic function is filtration, humidification and adjusting the temperature of the inspired air.

The lower respiratory tract includes: –Larynx which continues as the trachea, which divides and re-divides almost about 20 times. –At first, the trachea divides into 2 primary or main bronchi. –Each primary bronchus gives rise to secondary or lobar bronchi supplying the lobes of the lungs. –These again divide into tertiary or segmental bronchi, which supply segments of each lobe. –The tertiary bronchi divide into smaller airways called bronchioles. The smallest of such division is called terminal bronchioles. –The terminal bronchioles then divide into respiratory bronchioles and alveolar ducts. These passages finally terminate in dilated spaces called alveolar sacs, which open into alveoli. Each type of airway has its own features but there is a gradual change in structure.

Respiratory mucosa –Epithelium: pseudostratified columnar ciliated with goblet cells. –Lamina propria: loose C.T. containing: Rich blood vessels for warming of air. Lymphocytes for immune response. Seromucous (branched tubuloalveolar glands of Bowman) glands for secretion of mucous and absorption and detoxification of gases. Note: In some regions of nasal mucosa, there are veins resembling erectile tissue termed swell bodies. These are specialized veins which engorge periodically and alternatively to close one side of the nasal cavity, thus giving it time to recover from drying. This cyclic process is controlled by autonomic nerves.

Olfactory mucosa: –It is thicker than that of the respiratory mucosa and lacks the Goblet cells. –The glands in lamina propria are purely serous because the fluid secretion dissolves the odoriferous substances and also rapidly washed away to clear the receptors for new stimuli. –The lamina propria is rich in vascular plexuses. –The olfactory epithelium consists of 3 types of cells: Supporting cells: have narrow bases and broad apices that carry microvilli. The nucleus is just above the center of the cell. The cytoplasm contains lipofuscin pigments that give the area its yellow coloration. Bipolar olfactory cells: which carry the receptors of smell (bipolar neurons). The peripheral part is modified dendrite, which ends in a bulb called olfactory knob, which gives long non-motile cilia lying flat on the surface, and they carry the receptors for smell. The basal part is the axon, which passes through the basement membrane, and joins other axons forming the olfactory nerve that pass through the cribriform plate to reach the brain. Basal cells: short pyramidal with basal nuclei, they are undifferentiated and can give other types of the above cells.

In cross section, the trachea and the main bronchi are formed of 3 layers: Mucosa: –Epithelium: ciliated pseudostratified columnar epithelium with goblet cells. The cells rest on thick basal lamina. –Lamina propria: loose C.T. and elastic and reticular fibres along with lymphocytes. Submucosa: –Loose C.T. with seromucous glands. –Their long ducts project through the lumen. –Satellite shaped myoepithelial cells surround the acini and extend to ducts (mainly in the trachea). –The submucosa may contain aggregates of lymphoid tissue. Adventitia: –Contains "U"-shaped hyaline cartilage and dense C.T. containing blood vessels and nerves. –In the cartilage free parts, there are smooth muscles (trachealis).

Types of cells in lining of trachea and bronchi: 3 types of cells are seen by light microscope: –Ciliated columnar cells. –Goblet cells which produce mucoid secretion. –Basal cells: these are undifferentiated cells which can divide by mitosis and can give other types of cells. 2 types seen by electron microscope: –Brush cells which contain glycogen granules and carry microvilli. They may be either immature columnar cells or degranuled goblet cells or sensory cells because their bases make contact with nerve processes. –Granular cells which are of 2 types according to function of granules: Catecholamine secreting cells (neurosecretory cells). Protein hormone secreting cells (enteroendocrine like cells). They secrete amines and amine precursors.

Secondary bronchi The intrapulmonary secondary bronchi differs from the extrapulmonary primary bronchi by the following, otherwise the structure is the same: –The mucosa is folded. –The presence of muscularis mucosa which is composed of smooth muscle sheet beneath the lamina propria, so the mucosa is folded after fixation. –Adventitia contains discontinuous cartilage plates and dense C.T. The cartilage plates surround the bronchi leading to a circular appearance and not flattened at one side like the trachea and main bronchi. Note: Smaller generations of secondary bronchi are lined by ciliated simple columnar epithelium and their adventitia contains isolated smaller cartilages.

Bronchioles These are smaller in diameter. Its structure is as follows:- –Mucosa is folded and contains: ciliated simple columnar cells with goblet cells. –Muscularis mucosa: relatively thick smooth muscle layer. –Submucosa: no glands or lymphoid tissue. –Adventitia: loose connective tissue, no cartilage. –The bronchioles give rise to terminal bronchioles, which give respiratory bronchioles, that receives the alveoli. Terminal bronchioles These are the smallest branches of the conducting system. Their mucosa is characterized by: –The epithelium is cuboidal ciliated. –Absent goblet cells. –Presences of bronchiolar secretary cells called Clara cells. They are non-ciliated with projecting apex-carrying microvilli. They secrete surfactant, a phospholipid that alters the surface tension of the fluid layer covering the surface. The Respiratory bronchioles are lined with simple cuboidal ciliated epithelium, some of them are ciliated, and others are Clara cells.

Alveolar Ducts –The wall is deficient except in small areas lined by cuboidal cells in between the alveolar sacs. It looks like a long corridor along which open many lobbies (the alveolar sacs or antrum). The lobbies lead to the rooms (the alveoli). The Alveoli –The alveoli are lined by 2 types of cells: Flattened simple squamous cells (type I pneumocytes). Cuboid cells (type II pneumocytes) called secretory cells or septal cells that bulge into the alveolar lumen and contain multi-lamellar bodies. Their secretion is rich in phospholipids “surfactant” because it reduces the surface tension and prevents collapse of alveoli during expiration. Absence of the surfactant cells leads to respiratory distress syndrome (RDS). The spaces between the alveoli are called inter-alveolar septa, which consists of C.T. containing: –Fibres: reticular and elastic. –Septal cells (type II pneumocytes). –Other Cells: Fibroblasts, mast cells, leucocytes, macrophages. Other septal cells contain bundles of actin and myosin filaments which contract in response to hypoxia (their role is unknown). –Blood capillaries (continuous non-fenestrated); N.B.: the capillary endothelial cells and type I flat cells share fused basement membranes.

SUMMARY OF STRUCTURES OF THE RESPIRATORY TREE The structure of respiratory changes according to the functional need: –The air is inhaled through mouth or nose where it is humidified, warmed and the suspended particles trapped by the hair of nasal cavity and the mucous present on the mucosa. These particles are ultimately expelled out by the cilia. –As we go down the respiratory tree the cartilage decreases till it is completely absent in the bronchioles. In trachea it is present as a single semilunar plate while in secondary and tertiary bronchi, it is present in few discontinuous patches. –The height of the epithelium decreases down the tree till it becomes simple cuboidal in the terminal bronchioles. –The cilia decrease in number as we go down till they are absent in the terminal bronchioles. –The goblet cells disappear with the absence of cilia. –The smooth muscle increase in quantity till it is maximum in the bronchioles where it serves to act as a sphincter or control valve for regulation of air.

LOCAL DEFENSE MEHCHANISM –Alveolar Macrophages Present in the interalveolar septum and on alveoli, help in phagocytosis and disposal of antigens reaching the alveoli. Bacteria and viruses are easily phagocytosed and degraded  Indigested inert particles as carbon remain in macrophages for a long time and are deposited in islands of collagen (scar tissue).  Heavy exposure to asbestos, coal and other industrial particles, toxic gases also lead to their accumulation in scar tissues. Alveolar Fluid –This neutralizes bacteria and viruses by carrying the secretory IgA produced by B-lymphocytes (in local lymphoid tissue). There are also lymph nodes present in hila of the lungs, so both humoral and cell-mediated immune responses play a role in lung defense against infection. –Cigarette smoke interferes with the normal macrophage and immune function of the lung and increase susceptibility to infection.

CLINICAL NOTES –Absence of cartilage from the wall of bronchioles is a potential hazard, since these airways can constrict to a point of closing if the tone of their muscles is increased. This is the problem of asthma which is an allergic condition to non-specific lung irritant. Wheezing noises and difficulty in breathing occurs during expiration rather than inspiration. –Pulmonary Surfactant: Type II pneumocytes secrete phospholipid surfactant that decreases the alveolar surface tension forces to a minimal level thus preventing the alveoli from collapse. The presence of this secretion is important for the newborn to obtain their first breath of air. In premature delivery type II cells are immature. This leads to fatal respiratory difficulty in new born (respiratory distress syndrome).