Glossary of Functional Aspects of Lung Development
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- 6 Phases of Lung development:
- 1. Embryonic
2. Pseudoglandular - wks 4-7
3. Canalicular - wks 16-26
4. Terminal Sac - 26-birth
5. Alveolar - 32-8yrs
6. Postnatal - up to 8-12 yrs
- When does the Embryonic phase take place?
- Weeks 4-7
- What occurs in the embryonic phase?
- Respiratory Diverticulum forms and separates from forming esophagus
- What separates the Respiratory Diverticulum from the Esophagus?
- Tracheoesophageal Septum
- What components of the lungs form during the embryonic phase?
- Primary, Secondary, and Tertiary bronchi, and a few generations of subsegmental bronchi.
- What tissue development is lung devo like during the pseudoglandular phase?
- What is the main thing that happens in the Pseudoglandular phase?
- Dichotomous branching of bronchi - 20-25 generations
- What causes the dichotomous branching during the Pseudoglandular phase?
- Epithelial-mesenchymal interactions.
- What 2 types of interruptions can occur in the embryonic phase?
- 1. Anatomic
- What could an Anatomic interruption result in?
- -Lack of lungs
-Less/More lung tissue
- What could a Partitioning interruption result in?
- Tracheoesophageal Fissure
- What causes a Tracheoesophageal fissure?
- Inadequete partitioning due to abnormal or insufficient devo of the tracheoesophageal septum.
- What occurs in 90% of partitioning defects?
- Blind pouch at end of trachea
- Why is a tracheoesophageal fissure dangerous?
- Because it can cause regurgitation and aspiration pneumonia.
- What 2 major events occur in the Canalicular phase?
- 1. Distal branches of RD associate w/ devlpng capillaries
2. Type I/II Alveolar Cells begin to differentiate
- What is the limit of extrauterine survival? Why?
- Wks 22-24 - b/c this is when Type II pneumocytes begin to secrete surfactant
- What phase of lung devo correlates with wks 22-24?
- END of canalicular phase
- What is the main thing that happens in the Terminal Sac phase?
- Distal branches of bronchial tree dilate into Terminal sacs
i.e., the alveoli come in CONTACT with capillaries.
- What are Terminal Sacs?
- Primitive alveoli - they become subdivided into Alveoli
- What occurs in the Alveolar phase?
- Alveoli formation is accelerated and their septal walls thin.
- How much alveoli is formed
- Pre = about 20%
Post = The rest - majority
- What direction does each occur?
- FORMATION is proximal->distal
DIFFERNTN is distal->proximal
- What are the physical characteristics of the prenatal lung? (4)
- -Fluid filled
-Resp mvmnt is shallow/episodic
-High pulmonary vasc resistance
-Very low flow in pulm vessels
- What results from insufficient fluid production in the prenatal lung?
- Hypoplastic lungs
- At what 3 sites do changes at birth allow breathing to start?
- 1. Lung
- What changes occur in the lung at birth?
- Fluid is replaced by air
- How is fluid in the lungs replaced by air? (3 steps)
- 1. 1/3 expelled thru mouth
2. Most absorbed by lung vessels
3. 10% absorbed into lymphatics
- What major circulatory changes occur for breathing to start?
- -Closing of cardiac shunts
-Pulmonary vasc fills w/ blood
- What are the 2 cardiac shunts in the fetus?
- -Interatrial shunt
- What condition results from failure of fluid removal from the lung at birth?
- Wet lung
- What Neural changes at birth allow breathing to start?
- -Continuous resp movements
-Activation of Sympathetic Stimulation decreases secretory activity of lung epithelium
- Why is decreased secretory activity of the lungs good at birth?
- It allows the newly aerated lungs to not be refilled w/ fluid.
- What 3 sites contribute to fluid in the prenatal lung?
- 1. Amniotic fluid
2. Tracheal glands
3. Lung itself
- Through what 3 sites is lung fluid removed at birth?
- 1. Mouth (1/3)
2. Circulation (Majority)
3. Lymphatics (10%)
- Characteristics of the Alveoli air/liquid interface:
-Thin/compliant alveolar wall allows gas exchange
-Surface tension decreases surface area of alveolus
- How is surface area TENSION decreased in alveoli?
- Via surfactant
- What produces surfactant?
- Type II Pneumocytes
- When is surfactant produced at its peak in the prenatal infant?
- 35 weeks - surge in synthetic activity
- What does Surfactant accomplish?
- -Decreased surface tension
-Increased surface area
-Stablized alveolar surface
-Decreased fluid transfer btwn vasculature/alveolar surface
- What is pulmonary surfactant made up of?
- -Phospholipid 80-90%
- What 2 phospholipids contribute to surfactant?
- Phosphatidylcholine (lecithin)
- What is the major disease caused by abnormality in lung devolopment?
- Respiratory Distress syndrome
- 7 clinical signs of RDS:
- -Rapid breathing >60 breaths/min
-Subcostal chest retractions
-Rapid heart rate
-Anxious facial expression
- What are the 3 infants most at risk for RDS?
- -Preemies before 37 wks or below 1-1.5 kg
-Babies w/ uncontrolled diabetic moms
- Why are males prone to RDS more than females?
- Androgens are inhibitory to alveolar maturity
- When does RDS become apparent?
- 24-48 hrs after birth
- How is RDS treated?
- As a MEDICAL EMERGENCY
- What is an interchangeable term for RDS?
- Hyaline Membrane Disease (HMD)
- What are the 5 steps in the typical formation of hyaline membrane?
- 1. Injury to Type I cells
2. Breakdown of air/capillary interface
3. Leakage of serum into the alveolar lumen
4. Formation of a hyaline membrane
5. Disruption of surfactant
- What are the implications of hyaline membrane formation?
- The alveoli have decreased surface tension; so more work is required to inflate them at EVERY breath.
- How does Surfactant insufficiency manifest physiologically?
- -Poor lung compliance
-Atelactasis (alveoli collapse)
-Engorged pulmonary circulatn
- 2 Tests for detecting Pulmonary Surfactant Insufficiency:
Which is more accurate?
- -L/S ratio
-Phosphatidylglycerol - more accurate
- Principle of L/S ratio test:
- Sphingomyelin is constant
Lecithin should jump at 35 wks when surfactant production increases.
- What does each L/S ratio mean?
- >2 means 98% probably OK
1.5-2 means 50% chance RDS
<1.5 likely to have RDS (73%)
- What will enhance fetal lung maturation? (treatment if L/S ratio prior to delivery is low)
- Who is glucocorticoid administered to then? When?
-24 to 34 wks gestation optimal
-24-48 hrs before birth
- Partitioning defects in lung devo can occur up to what stage?
- Pseudoglandular - when the last 20-25 branches of the bronchial tree grow.
- What anomalies are included in the classification "Partitioning defects"?
- -Tracheoesophageal fistula
-Lack of lung
- How are the Canalicular and Terminal Sac periods different?
- Terminal Sac starts extrauterine viabilty b/c the capillaries are not only ASSOCIATED w/ alveolar cells, but in CONTACT.
- What is the ACTUAL limit of extrauterine viability?
- 22-26 weeks - at the END of the canalicular phase
- What is a much better time to be born prematurely?
- 35 weeks - b/c Type II pneumocyte production greatly increases at that time
- What is tracheoesophageal fistula usually associated with?
- Esophageal atresia
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