Respiratory System

• Components of the respiratory system

  • Conducting portion
  • Respiratory portion

  
• Conducting portion


  Consists of the nasal cavities, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles

  
• Respiratory portion


  Where the system's main function of gas exchange occurs, consisting of respiratory bronchioles, alveolar ducts, and alveoli
• Components of the nasal cavities

  • External, dilated vestibule
  • Internal nasal cavity

  
• Nasal vestibule


  • Skin enters the nares (nostrils) partway, includes sweat glands, sebaceous glands, and coarse, moist vibrissae (hairs) which filter out particulate material from inspired air
  • Epithelium loses its keratinized nature and undergoes a transition to typical pseudostratified columnar epithelium which also lines the nasal cavities
• Nasal cavities


  Lie within the skull as two cavernous chambers separated by the osseous nasal septum, with three bony shelflike projections called conchae or turbinate bones extending from each lateral wall

  
• The mucosa covering the nasal cavity walls has a lamina propria with important roles in conditioning inhaled air
• Seromucous glands


  The thin mucus layer produced by these glands and goblet cells serves to trap particulate and gaseous air impurities that are then removed, and the secretions also contain immunoglobulin A (IgA) from plasma cells in the lamina propria
• Cell types in respiratory epithelium

  • Ciliated columnar cells
  • Goblet cells
  • Brush cells
  • Small granule cells (or Kulchitsky cells)
  • Basal cells
• Respiratory epithelium


  Covers the middle and inferior conchae, most of the nasal cavities and conducting portion of the respiratory system

  
• Olfactory epithelium


  A specialized region of the mucous membrane covering the superior conchae at the roof of the nasal cavity, containing olfactory neurons, supporting cells, and basal cells
• Olfactory neurons


  • Bipolar neurons present throughout the olfactory epithelium, with axons that leave the epithelium and unite in the lamina propria as very small nerves that then pass to the brain through foramina in the cribriform plate of the ethmoid bone to form the olfactory nerve (cranial nerve I)
• Supporting cells in olfactory epithelium

  • Columnar, with narrow bases and broad, cylindrical apexes containing the nuclei and extending microvilli into the fluid layer, help maintain a microenvironment conducive to olfactory function and survival
• Basal cells in olfactory epithelium

  • Small, spherical, or cone-shaped cells near the basal lamina, serve as stem cells for the olfactory neurons and support cells
• Olfactory glands (of Bowman)


  Large serous glands in the lamina propria of the olfactory epithelium that produce a constant flow of fluid surrounding the olfactory cilia and facilitating the access of new odoriferous substances
• Loss of the sense of smell due to toxic fumes or physical injury to the olfactory mucosa itself is usually temporary due to the regenerative activity of the epithelial stem cells
• Paranasal sinuses


  Bilateral cavities in the frontal, maxillary, ethmoid, and sphenoid bones of the skull, lined with a thinner respiratory epithelium having fewer goblet cells

  
• The paranasal sinuses communicate with the nasal cavities through small openings, and mucus produced there is moved into the nasal passages by the activity of the ciliated epithelial cells

  
• Chronic sinusitis and bronchitis are components of primary ciliary dyskinesia, or Kartagener syndrome, an inherited genetic disorder characterized by defective ciliary action
• Nasopharynx


  The first part of the pharynx, which the nasal cavities open into posteriorly

  
• Oropharynx


  The posterior part of the oral cavity leading to the larynx and esophagus, continuous with the nasopharynx

  
• Larynx


  • A short (4 cm × 4 cm) passage for air between the pharynx and the trachea, with a rigid wall reinforced by hyaline cartilage and smaller elastic cartilages connected by ligaments, movements of which by skeletal muscles participate in sound production during phonation

  
• Epiglottis


  A flattened structure projecting from the upper rim of the larynx that serves to prevent swallowed food or fluid from entering the laryngeal passage
• Vocal folds (or cords)

  • The lower pair of folds in the larynx, covered with nonkeratinized stratified squamous epithelium and containing a dense regular bundle of elastic connective tissue (the vocal ligament) and large bundles of striated fibers comprising the vocalis muscle that allow each vocal fold to be moved
• Inflammation of the larynx, or laryngitis, is typically due to viral infection and is usually accompanied by edema or swelling of the organ's lamina propria, changing the shape of the vocal folds or other parts of the larynx and producing hoarseness or complete loss of voice

  
• Trachea


  10-12 cm long in adults, lined with typical respiratory mucosa in which the lamina propria contains numerous seromucous glands producing watery mucus, with a series of about a dozen C-shaped rings of hyaline cartilage between the submucosa and adventitia reinforcing the wall and keeping the tracheal lumen open

  
• Coughing is a reflex action produced most often by viral infection or other irritation of the trachea or other region of the respiratory tract
• Bronchial tree and lung

  The trachea divides into two primary bronchi that enter each lung at the hilum, which then branch into secondary (lobar) bronchi, tertiary (segmental) bronchi, and progressively smaller bronchi and bronchioles
• Bronchi


  • The mucosa of the larger bronchi is structurally like the tracheal mucosa except for the organization of cartilage and smooth muscle, with cartilage rings becoming smaller and more isolated plates as bronchial diameter decreases, and lymphocytes and lymphoid nodules becoming more abundant
• Bronchioles


  The intralobular airways with diameters of 1 mm or less, formed after the tertiary bronchi
• Bronchi


  • Each primary bronchus branches repeatedly, with each branch becoming progressively smaller until it reaches a diameter of 1-2 mm
  • The mucosa of the larger bronchi is structurally like the tracheal mucosa except for the organization of cartilage and smooth muscle
  • In the primary bronchi most cartilage rings completely encircle the lumen, but as the bronchial diameter decreases, cartilage rings are gradually replaced with smaller isolated plates of hyaline cartilage
  • Numerous lymphocytes are found both within the lamina propria and among the epithelial cells
  • Lymphatic nodules are present, especially at the branching points of the bronchial tree
  • Mucosa-associated lymphoid tissue (MALT) also becomes relatively more abundant as bronchi become smaller and the cartilage and other connective tissue are reduced
• Bronchioles


  • Bronchioles are the intralobular airways with diameters of 1 mm or less, formed after about the tenth generation of branching
  • They lack both mucosal glands and cartilage, although dense connective tissue is associated with the smooth muscle
  • In the larger bronchioles, the epithelium is still ciliated pseudostratified columnar, but this decreases in height and complexity to become ciliated simple columnar or simple cuboidal epithelium in the smallest terminal bronchioles
  • The ciliated epithelial lining of bronchioles begins the mucociliary apparatus or escalator, important in clearing debris and mucus by moving it upward along the bronchial tree and trachea
  • The cuboidal epithelium of terminal bronchioles consists largely of club cells or bronchiolar exocrine cells, with nonciliated, dome-shaped apical ends containing secretory granules
  • These exocrine cells have various functions, including secretion of surfactant lipoproteins and mucins, detoxification of inhaled xenobiotic compounds, and secretion of antimicrobial peptides and cytokines for local immune defense
  • Also present in the terminal bronchiole epithelium are chemosensory brush cells and DNES small granule cells
• Bronchiolar lamina propria


  • Contains elastic fibers and smooth muscle, producing folds in the mucosa
  • Muscular contraction in both the bronchi and the bronchioles is controlled primarily by nerves of the autonomic nervous system
• Respiratory bronchioles


  • Each terminal bronchiole subdivides into two or more respiratory bronchioles, which always include saclike alveoli and represent the first part of the respiratory region

  
• Alveolar ducts


  • Distal ends of respiratory bronchioles branch into tubes called alveolar ducts, which are completely lined by the openings of alveoli
  • Larger clusters of alveoli called alveolar sacs form the ends of alveolar ducts distally and occur occasionally along their length
  • The lamina propria is now extremely thin, consisting essentially of a web of elastic and reticular fibers that encircles the alveolar openings and closely surrounds each alveolus
  • Prominent in this sparse connective tissue, a network of capillaries also surrounds each alveolus

  
• Alveoli


  • Alveoli are saclike evaginations, each about 200 μm in diameter, from the respiratory bronchioles, alveolar ducts, and alveolar sacs
  • Along with the airways, alveoli are responsible for the spongy structure of the lungs
  • Air in the alveoli is separated from capillary blood by three components referred to collectively as the respiratory membrane or blood-air barrier: two or three highly attenuated, thin cells lining the alveolus; the fused basal laminae of these cells and the endothelial cells of capillaries; and the thin capillary endothelial cells
  • O2 from the alveolar air diffuses through the blood-air barrier into the capillary blood and binds hemoglobin in erythrocytes; CO2 diffuses into the alveolar air from the pulmonary blood

  
• Type I alveolar cells

  • Also extremely attenuated cells lining the alveolar surfaces
  • Type I cells constitute the alveolar side of the blood-air barrier and make up about 95% of the alveolar lining

  
• Type II alveolar cells

  • Cuboidal cells bulging into the air space, interspersed among the type I alveolar cells, and bound to them with tight junctions and desmosomes
• Alveolar macrophages


  • Also called dust cells, are found in alveoli and in the interalveolar septum
  • Tens of millions of monocytes migrate daily from the microvasculature into the lung tissue, where they phagocytose erythrocytes lost from damaged capillaries and airborne particulate matter that has penetrated as far as alveoli
  • Active alveolar macrophages can often be distinguished from type II pneumocytes because they are slightly darker due to their content of dust and carbon from air and complexed iron (hemosiderin) from erythrocytes
  • Filled macrophages have various fates: most migrate into bronchioles where they move up the mucociliary apparatus for removal into the esophagus, others exit the lungs in the lymphatic drainage, and some remain in the interalveolar septa connective tissue for years
• Lung vasculature and nerves

  • Blood circulation in the lungs includes both the pulmonary circulation, carrying O2 -depleted blood for gas exchange, and bronchial circulation, carrying O2 -rich blood
  • Both parasympathetic and sympathetic autonomic fibers innervate the lungs and control reflexes regulating smooth muscle contractions which determine the diameters of the airways
  • General visceral afferent fibers, carrying poorly localized pain sensations, are also present
  • The nerves are found primarily in the connective tissue surrounding the larger elements of the bronchial tree and exit the lung at the hilum