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Respiratory Tract To Lungs

  1. Nasal cavity
    1. Divided by a hyaline-cartilage nasal septum in the midline.
    2. Stratified squamous epithelium (hairy) of the nares changes to
    3. A lining nasal mucosa of
      1. Pseudostratified, columnar, ciliated epithelium with mucus-secreting goblet cells
      2. A loose lamina propria, with many leucocytes, blood vessels, and mixed muco-serous glands.
    4. Venous plexuses, to warm the air, underlie the epithelium.
    5. Turbinate bones in the conchae support the mucosa.
    6. A small part of the mucosa is olfactory, with a neuroepithelium and Bowman’s glands.
    7. Paranasal air sinuses open off the main cavity.
    8. The folded pharyngeal tonsil, covered by pseudostratified, columnar, ciliated epithelium, lies posteriorly in the pharynx.
  2. Larynx
    1. Hollow chamber, whose walls are supported by cartilages, connected by ligaments and membranes, and moved by skeletal muscles.
    2. The extrinsic and intrinsic muscles move the larynx up and under the epiglottis in swallowing, and move the cartilages and tense the vocal cords during phonation and breathing.
    3. The cartilages are hyaline tending to calcification, or elastic for the epiglottis, cuneiforms, comiculates, and the apices and vocal processes of the arytenoids.
    4. Mucosa is mostly pseudostratified, columnar, ciliated epithelium with goblet cells, on a loose lamina propria rich in elastic fibres, mucous and mixed glands, leucocytes and sometimes lymphoid nodules.
    5. Two constrictions occur: the false vocal cords/ventricular folds; and the lower, true, cords. The true vocal chords are elastic ligaments tensed by the adjacent vocalis muscle, and are covered with stratified squamous epithelium*. There are no glands in their lamina propria.
    6. The epiglottis, too, has stratified squamous epithelium on its exposed tip and upper surface.
  3. Trachea
    1. Flexible, extensible tube, with an always-patent lumen.
    2. Mucosa as for the larynx and the cilia sweep towards the pharynx, but the elastic fibres run longitudinally as a layer between mucosa and submucosa.
    3. Supporting C-shaped pieces of hyaline cartilage are incomplete on their oesophageal side.
    4. The gap in the C is crossed by trachealis smooth muscle and CT.
    5. Outer adventitia is fibro-elastic CT.


  1. Bronchial tree serving the lungs
    1. Primary bronchi branch to form the
    2. Intrapulmonary lobar bronchi, branching to form segmental bronchi, then lobular bronchioles. After about 9-12 generations of branching, bronchioles replace bronchi.
    3. Terminal bronchioles lead to respiratory bronchioles, off which open the respiratory exchange units, and not just at the end, but along the bronehiole. [For efficiency, the branching, tubular architecture of air conductance overlaps slightly the honeycomb architecture of gas exchange.]
    4. Bronchi resemble the trachea in structure, except that the cartilage pieces in the wall have very irregular shapes, and the smooth muscle forms a nearly complete layer - muscularis mucosae - between the cartilages and the lumen.
    5. Bronchioles are smaller than bronchi:
    6. They have no cartilages;
    7. Their elastic fibres merge with those of the surrounding lung tissue ;
    8. The epithelium changes to simple, low ciliated columnar with a few goblet cells
    9. No mucous glands are present in the lamina propria, where the smooth muscle is relatively substantial.
    10. Sharing the connective tissue of the branching bronchi are blood vessels, nerves and lymphatic vessels, entering or leaving at the hilum or lung root.
    11. Hilar structures include arteries (bronchial and pulmonary), veins, lymphatics (from two systems), bronchi, lymph nodes, ganglia, nerves (to bronchial, bronchiolar, and vascular smooth muscles; and sensory), and adipose and other CT.
    12. The carotid body-like glomus pulmonale in the pulmonary artery’s adventitia is of uncertain function.
  2. Mucosa of the lower airway
    1. Cell types in the epithelium:
      1. Ciliated columnar cells, with lysosomes and some microvilli;
      2. mucus-secreting goblet cells;
      3. Basal ‘undifferentiated’ cells to replace the specialized kinds;
      4. Clara’s non-ciliated bronchiolar secretory cells with granules and GER;
      5. Neuroendocrine cells;
      6. Lymphocytes migrated from the lamina propria.
    2. A sheet of sticky mucus is moved by ciliary action over the mucosa to catch and remove particles - the mucociliary escalator.
    3. The basal lamina typically is thick.
    4. Muco-serous mixed glands, where present in the lamina propria, are small, compound tubular, and respond under nervous control to irritant stimuli, e.g. smoke.
  3. Respiratory chambers
    1. Respiratory bronchiole has simple, low columnar or cuboidal bronchiolar and ciliated cells elastic fibres and smooth muscle support the epithelium‘s BL.
    2. Opening out along the respiratory bronchiole are alveoli, whose openings are ringed by smooth muscle.
    3. At the end of the respiratory bronchiole are one or more long alveolar ducts.
    4. Alveolar ducts can be viewed as being three to six atria, vestibules, leading to alveolar sacs, made up of varying numbers of alveoli.
    5. One alveolus or cubicle shares an alveolar wall with the ones adjacent and backing on to it. The wall is thus interalveolar and carries the many capillaries, whose blood is to receive oxygen and give up carbon dioxide.
    6. Angiotensin converting enzyme in pulmonary capillaries cleaves angiotensin I to make it the potent angiotensin II.
  4. Interalveolar wall
    1. Air side - continuous alveolar epithelium with:
      1. Type I pneumocytes/squamous cells (and
      2. Pneumocytes type II/septal or great alveolar cells with prominent lipid cytosomes/ multilamellar bodies in their cytoplasm.

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