LEC 18: Respiratory System - 08.28.14 Flashcards
Upper vs. Lower Respiratory Tract
- Upper
- nasal cavities, etc.
- Lower
- larynx
- trachea
- bronchi
- lungs
When do the lower respiratory organs begin to form
~4 weeks
Respiratory primordium
- respiratory primordium indicated by median outgrowth from caudal end of ventral wall of primordial pharynx
- elongates to form laryngotracheal tube
Laryngotracheal groove
- median outgrowth in anterior wall of endodermally-derived foregut
- by end of 4th week the laryngotracheal groove invaginates to form pouch-like respiratory diverticulum (lung bud)
Formation of lung bud
- by end of 4th week the laryngotracheal groove invaginates to form pouch-like respiratory diverticulum (lung bud)
- as diverticulum elongates, it is invested with splanchnic mesenchyme and its distal end enlarges to form tracheal bud
Relationship between respiratory diverticulum and primordial pharynx
- respiratory diverticulum separates from primordial pharynx, but maintains communication with it through the primordial laryngeal inlet
composition of laryngeal tube
endoderm = epithelium and glands of larynx, trachea, bronchi, and pulmonary epithelium
splanchnic mesoderm = connective tissue, cartilage, smooth muscle
functions of larynx
- swallowing
- respiration
- voice production
upper border = epiglottis
lower border = cricoid cartilage
Formation of larynx
- forms at cranial end of LT tube
- epithelial lining develops from endoderm of LT tube
- laryngeal cartilages develop from mesenchyme that is derived from neural crest cells
- mesenchyme at cranial end of LT tube proliferates rapidly, forms paired arytenoid swellings
- swellings grow toward tongue, forming T-shaped laryngeal inlet
- Proliferation of epithelium temporarily occludes laryngeal lumen (recanalizes by week 10)
recanalization of larynx
- T-shaped glottis bounded by lateral arytenoid swellings and cranial epiglottis
- temporarily occludes laryngeal lumen
- recanlized at week 10
vocal cords
vocal cords form from laryngeal ventricles (recesses)
epiglottis
- covers larynx
- develops from caudal hypobranchial eminence
- muscles form from myoblasts in 4th-6th arches
laryngeal web
- incomplete recanalization of larynx in week 10
- partial obstruction of airway in newborn
- not necessarily fatal, may be fixed
development of trachea (respiratory diverticulum)
- straight portion of respiratory diverticulum is the primordium of the trachea
- bronchial buds develop into bronchial tree
What controls the extent of branching within the respiratory tract
- mesoderm surrounding endoderm controls the extent of branching within the respiratory tract
- endoderm = epithelium and glands of trachea and pulmonary epithelium
- mesoderm = tracheal cartilage, connective tissue and muscles
laryngeal-tracheal diverticulum
- by end of week 4, LT groove envaginates to form laryngeal-tracheal diverticulum
- respiratory diverticulum soon separates from the primordial pharynx, however it maintains communication with it through the primordial laryngeal inlet
Laryngeal inlet
- maintains connection between laryngeotracheal diverticulum and primordial pharynx
tracheo-esophageal septum
- longitudinal folds develop in laryngeal-tracheal diverticulum
- folds fuse to form tracheo-esophageal septum
- divides laryngealtracheal tube from oro-pharynx and esophagus
T-E fistula (tracheoesophageal fistula)
- abnormal communication between trachea and esophagus
- most common anomaly of lower respiratory tract
- incomplete fusion of TE folds
- 85% associated with esophageal atresia (poor esophagus development)
bronchi and lungs
- trachea divides into L/R bronchi
- each bronchus divides again and again, narrowing
- smallest airways end in aveoli (thin air sacs, look like balloons)
- tiny blood vessels surround the 300 million aveoli in lungs
- oxygen picked up from blood vessels
- CO2 dumped (breathed out)
primary bronchus vs. secondary bronchi
- 5th week, connection of each bronchial bud with the trachea enlarges to form primordium of main bronchus
- R is slightly larger than L bronchus and oriented more vertically (why you get stuff stuck in right side)
Describe the differences between R/L bronchi
RIGHT
- 3 secondary bronchi
- larger, shorter, more vertical
LEFT
- 2 secondary bronchi
- 2 lobes
segmental bronchi
- form at 7 weeks
- bronchopulmonary segment = bronchus + mesenchyme
- by 24 weeks, 17 orders of branches
Maturation of the lungs (in 4 stages)
- pseudoglandular phase (5-17 weeks)
- canalicular period (16-25 weeks)
- terminal sac period (24 weeks until birth)
- alveolar period (birth-8 years)
pseudoglandular phase
- 5-17 weeks
- incompatible with life
- formation and growth of duct systems
- bronchopulmonary segments not well developed
canalicular phase
- 16-25 weeks
- late canalicular phase may be compatible with life (respiration possible, especially towards end)
- highly vascular
- alveolar sacs not truly differentiated
terminal sac phase
- 24 weeks until birth
- capillaries bulge into alveoli
- epithelium very thin
- contact between epithelial and endothelial cells permits gas exchange
- proliferation of capillary network
- Type 1 alveolar cells line terminal saccules
- Type 2 alveolar cells secrete surfactant
- forms monomolecular film over internal walls of terminal saccules
- lowers surface tension
- reaches adequate level in late fetal period
alveolar phase
- birth to 8 years
- terminal sacs have very thin epithelial lining
- alveolocapillary membrane allows gas exchange
- terminal saccules = future air ducts
surfactant
- Type 2 alveolar cells secrete surfactant
- forms monomolecular film over internal walls of terminal saccules
- lowers surface tension
- reaches adequate level in late fetal period
Type 1 vs. Type 2 alvelolar cells
Type 1 alveolar cells = line terminal saccules
Type 2 alveolar cells = secrete surfactant
Lungs prior to 26-28 weeks
- respiratory distress
- inadequate surfactant
- insufficient alveolar surface area
- inadequate pulmonary vasculature
requirements for autonomous gas exchange
- surfactant
- transformation of lungs to gas exchanging organ
- establishment of parallel pulmonary and systemic circulations
- 95% of alveoli develop post-natally
preparation for respiration
- fetal breathing movements
- condition respiratory muscles
- stimulate lung development
- at birth, lungs filled with fluid
- birth
- replacement of intra-alveolar fluid with air
normal lung development in-utero depends on…
- adequate thoracic space for lung growth
- fetal breathing movements
- amniotic fluid volume
respiratory distress syndrome
- 2% of live newborns
- lack of surfactant production
- lungs are under inflated
- prenatal steroids
- produce exogenous (outside) surfactant
