HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 22 RESPIRATORY TRACT

The tract rhythmically expels spent air and takes in fresh through conditioning passages, conducting it to the respiratory portion of the lungs, where the walls of the air-filled chambers are thin enough to permit an exchange of gases between blood and air. The respiratory movements involve chemoreceptors, brain centres, the thoracic cage, and various muscles: these structures belong, together with the respiratory tract, in the respiratory system Powerpoint. The lungs also have important metabolic functions not directly related to gas exchange, e.g., the activation of circulating angiotensin I, and the inactivation of some other vasoactive agents.

A RESPIRATORY TRACT TO LUNGS

l Nasal cavity
l 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:
.. (a) pseudostratified, columnar, ciliated epithelium with mucus-secreting goblet cells, on
.. (b) 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 (Chapter l2.B.5.la) 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.

9 Nasal functions:

2 Larynx
l 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, corniculates, 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
l 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.

B LUNGS

The structure of the lungs reflects the way in which the air is moved: l Bronchial tree serving the lungs
l Primary bronchi branch to form the
2 intrapulmonary lobar bronchi, branching to form segmental bronchi, then lobular bronchioles. After about 9-l2 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 bronchiole. [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:
.. they have no cartilages;
.. their elastic fibres merge with those of the surrounding lung tissue;
.. the epithelium changes to simple, low ciliated columnar with a few goblet cells;
.. no mucous glands are present in the lamina propria, where the smooth muscle is relatively substantial.
6 Sharing the connective tissue of the branching bronchi are blood vessels, nerves and lymphatic vessels, entering or leaving at the hilum or lung root.
7 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.
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:
  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
l 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.
Processing distortions in lung slides often make the atria and sacs hard to make out.
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
l Air side - continuous alveolar epithelium with:
.. (a) type I pneumocytes/squamous cells; and
.. (b) pneumocytes type II/septal or great alveolar cells, with prominent lipid cytosomes/ multilamellar bodies in their cytoplasm.
2 Surfactant is a stabilizing fluid film of lipids (90%) and proteins (10%), covering the epithelium and lowering surface tension. The principal surface-active agent is the lipid, dipalmitoyl phosphatidylcholine (DPPC). The type II cells synthesize this film, but also are the stem cell to replace themselves and Type I cells. Cytosomes are stored surfactant.
3 Alveolar macrophages/dust cells lie free in the alveoli.
4 Alveolar epithelium lies on a basal lamina sometimes merging with, and sometimes separated from, the
5 basal lamina of a blood capillary, on which lies an
6 unfenestrated endothelium on the blood side.
7 Where the two basal laminae are separated, the space - zona diffusa - is taken by elastic and reticular fibres, fibroblasts, macrophages and other CT cells.
8 The pulmonary blood-air barrier can therefore be as thin as 300 nm, and has a very extensive area.
9 Communication between adjacent alveolar sacs is through holes in the wall - alveolar pores.
l0 Basal laminae, fibres, and surfactant maintain the shape and patency of alveoli during respiration.

5 Pleurae are fibro-elastic vascular membranes with mesothelial coverings. From the visceral pleura, CT septa run in to subdivide the lung into lobules and carry lymphatic and venous vessels.

C RESPIRATORY TRACT

l Development
l From an endodermal bulge on the foregut, which gives the trachea, then two buds for the bronchi and lungs.
2 Continued budding and branching, and enclosure of the hollow buds by mesenchyme, produce a system of cuboidal epithelium-lined tubules with surrounding differentiating CT and vessels.
3 Early development thus is analogous to that of a compound exocrine gland, until the later phase, when the pulmonary alveoli form. Inadequately developed alveoli, with no surfactant, are a major hazard of premature birth.
4 Surfactant comprises lipids, and surfactant glycoproteins SP-A, -B, -C, & -D, which variously cause the lamellar material to become a monolayer, enhance the lowering of surface tension, stabilise the lipids, and modify host defences.
5 For the development of glands and the lung, complex mesenchymal-epithelial inductive (instructional) interactions occur, and recur during repair and tumour development.

2 Respiratory protective mechanisms

  1. Secretion of entrapping mucus by goblet cells and mixed glands,
  2. which is swept pharynx-wards by the ciliary beating action.
  3. Solitary lymphoid nodules and tonsils, and their lymphocyte progeny, for immune defence.
  4. Phagocytic alveolar macrophages/dust cells.
  5. Reflex coughing, sneezing, and constriction of bronchioles.
  6. Secretion of serous bacteriolytic materials, e.g., defensins and lysozyme.
  7. Upper airway recovers water and heat, preventing too much loss in the expired air.

Some protection is hazardous in that enzymes from WBCs can break down elastin; and activated lung macrophages stimulate fibroblasts to lay down movement-restricting collagen - an interstitial fibrosis.

Various defects in the arms and microtubules of cilia (primary ciliary dyskinesia) can prevent proper clearance and cause recurrent lung infection. Affected men are often infertile from an accompanying paralysis of sperm.


William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159