Respiratory System

1
Respiratory System

• At about 4 weeks the respiratory diverticulum
  (lung bud) appears
• Outgrowth from the ventral wall of the foregut
• Epithelium lining from endoderm
• Cartilaginous muscular components derived from
  splanchnic mesoderm
• Initially lung bud communicates with foregut but
  will separate as esophagotracheal ridges fuse
  ?esophagotracheal septum
• Respiratory primordium communicates with pharynx
  via laryngeal orifice

2
Larynx

• Internal lining from endodermal origin
• Cartilages muscles originate from mesenchyme of
  4th 6th pharyngeal arches
• All muscles innervated by braches of vagus (X)
• As cartilages form, the laryngeal epithelium
  proliferates rapidly temporarily occluding lumen
• Subsequently vacuolization recanalization
  occur-a pair of lateral recesses form ?laryngeal
  ventricles
• Bounded by folds of tissue that differentiate
  into false true vocal cords

3
Branches of X CN

• Superior laryngeal nerve
• Innervates derivatives of the 4th pharyngeal arch
• Recurrent laryngeal nerve
• Innervates derivatives of the 6th pharyngeal arch

4
Trachea, Bronchi, Lungs

• During its separation from foregut the lung bud
  forms the trachea two lateral outpocketings
  (bronchial buds)
• At beginning of the 5th week each bud enlarges ?
  forming right left main bronchi
• Right ? forms 3 secondary bronchi
• Will give rise to 3 lobes
• Left ? forms 2 secondary bronchi
• Will give rise to 2 lobes

5
Trachea, Bronchi, Lungs (cont.)

• With subsequent growth in both caudal lateral
  directions the lung buds penetrate into the
  coelomic cavity
• Space is narrow ? pericardioperitoneal canal
• Gradually filled by expanding lung buds
• Ultimately the pericardioperitoneal canals are
  seperated from the peritoneal pericardial
  cavities by the pleuroperitoneal
  pleuropericardial folds

6
Pleural cavity (space)

• Mesoderm which covers the outside of the lung
  develops into the visceral pleura
• Somatic mesoderm layer, covering the body wall
  from the inside becomes the parietal pleura
• The space between the above is the pleural
  cavity/space
• After birth a negative pleural pressure (vacuum)
  keeps the lungs inflated against the chest wall

7
Secondary bronchi

• During further development secondary bronchi
  divide repeatedly in dichotomous fashion forming
• 10 tertiary (segmental) bronchi in right lung
• 8 tertiary (segmental) bronchi in left lung
• Creation of the bronchopulmonary segments of the
  adult lung
• By the end of the 6th month approximately 17
  generations of subdivisions have formed
• Additional 6 divisions will form postnatally

8
Maturation of the lungs

• Up to the 7th month the bronchioles divide
  continuously into more smaller canals
  vascular supply ? steadily
• Respiration becomes possible when some cuboidal
  respiratory bronchiole cells ?
• Into thin flat cells which are associated with
  numerous blood lymph capillaries
• Surrounded spaces ? terminal sacs or primitive
  alveoli
• During 7th month sufficient capillaries are
  present lung is mature enough allowing the
  premature infant to survive

9
Maturation of lungs (cont.)

• In the last two months of prenatal life for
  several years postnatally the of terminal sacs
  (alveoli) ?
• Alveolar epithelial cells lining the sacs
• Type I (majority)
• Become thinner allowing capillaries to protrude
  into sacs ? respiratory membrane
• Type II (minority)
• Develops at end of 6th month
• Produces surfactant
• phospholipid which ? surface tension

10
Maturation of lungs (cont.)

• Before birth the lungs are filled with fluid
• High Chloride concentration
• Little protein
• Some mucus from bronchial glands
• Surfactant from type II alveolar epithelial cells
• Amount ? especially during last 2 weeks before
  birth
• Fetal breathing movements begin before birth
  cause aspiration of amniotic fluid
• Movements are important
• lung development
• Condition respiratory muscles

11
Respiration at birth

• When respiration begins at birth
• Most of lung fluid rapidly resorbed by
• Blood
• Lymph capillaries
• Small amount expelled via bronchi trachea
  during delivery
• After fluid is resorbed surfactant remains
  deposited as a thin phospholipid coat on the
  alveolar cell membranes
• preventing a water-air interface during breathing
  thus keeping surface tension minimized
• Without surfactant alveoli would collapse during
  expiration (atelectasis)

12
Alveoli

• Estimated that only 1/6 of adult number of
  alveoli are present at birth
• Remaining alveoli are formed during first 10
  years postnatally through the continuous
  formation of new primitive alveoli
• Growth of lungs after birth is 1o due to an ? in
  the of respiratory bronchioles alveoli, not
  an ? in size of the alveoli

13
Congenital defects

• Tracheoesophageal fistula
• Can be associated with polyhydramnios
• Amniotic fluid may not pass to stomach
  intestines
• Gastric contents and/or amniotic fluid may enter
  trachea through a fistula
• Causing pneumonitis and pneumonia
• Congenital cysts
• Formed by dilation of terminal or larger bronchi
• May be small multiple giving the lung a
  honeycombed appearance on X-ray
• May be restricted to one or more larger ones
• Drain poorly frequently cause chronic infections

14
Congenital defects (cont.)

• Absence of lungs
• Rare, not compatible with life
• Agenesis of one lung
• Rare, compatible with life
• Abnormal divisions of the bronchial tree
• More common
• Supernumerary lobules
• Little functional significance, but may cause
  unexpected difficulties in bronchoscopy
• Ectopic lung lobes
• May arise from trachea or esophagus from
  additional respiratory buds

15
Respiratory Distress Syndrome a.k.a hyaline
membrane disease

• Lack of surfactant in premature infant
• Risk that the alveoli will collapse during
  expiration causing respiratory distress synd.
• Common cause of death in premature infants
• 20 of all infant deaths in newborn period
• Partially collapsed alveoli contain fluid with
  high protein content, many hyaline membranes,
  lamellar bodies probably from surfactant layer
• Treatment with artificial surfactant
  glucocorticoids to surfactant production have
  significantly ? mortality