RESPIRATORY SYSTEM

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RESPIRATORY SYSTEM
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• 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)

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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).

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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).