1. Chapter 8 The Respiratory System

2. Objectives
State the major developmental events of the respiratory system in the womb.
Describe the key elements of normal fetal circulation.
State what happens to the respiratory system at birth.
Describe the developmental events in the respiratory system that continue after birth.

4. Development of the Respiratory System
Extends from almost conception into childhood.
During canalicular phase, life becomes possible.

6. Fetal Lung Development
At end of canalicular phase, primitive acini form, covered with type I and II pneumocytes.
Life viable if airway, MV, surfactant provided
During terminal saccular stage more acini form.
There is thinning of the type I cells
Type II cells mature and produce surfactant.
Alveolar stage begins at about 32 weeks.
Mature alveoli/capillary membranes appear.
At birth, about 50 million alveoli are present.
By age 8, there will be about 300 million.
MV – Mechanical Ventilation See Table 8-1 for weeks associated with each stage and level of maturation

12. The Fetal Lung Lung maturation is determined by surfactant.
Phosphatidylcholine levels predictive
Lecithin/sphingomyelin ratio (L/S ratio)
Phosphatidylglycerol (PG) concentration
Fetal lung fluid is constantly produced
Slight positive pressure keeps lungs inflated.
Promotes normal lung development
At birth, lungs hold about 40 ml of fluid.
If deficient, can result in hypoplastic lung

13. Uterine Life
In utero life depends on placental structure, which provides, among many things:
Gas exchange
Nutrients and waste removal
Defense against disease
Fetal circulation incorporates the placenta by the umbilicus and use of three special shunts:
Ductus venosus, ductus arteriosus, and foramen ovale

15. Uterine Life (cont.)

16. Fetal Circulation
Placenta large volume, low resistance system, so fetal SVR is low
Umbilical vein returns oxygenated blood from the placenta to fetus via the ductus venosus.
Flows into the IVC and on to the RA
Oxygenated blood is preferentially shunted through the foramen ovale from the right to the left atrium.
Provides oxygenated blood to systemic circulation

17. Fetal Circulation (cont.)
In utero fetal lungs have high PVR due to low PAO2.
Ductus arteriosus shunts blood from high-resistance pulmonary artery to low-resistance aorta.
PAO2 – Alveolar PO2

18. Cardiopulmonary Events at Birth
Fetal lung fluid
Prior to birth, production stops and absorption starts.
One third of fluid is expelled by vaginal squeeze.
Pulmonary lymphatics absorb remaining fluid.
Tactile and thermal stimuli initiate first breath.
Initial breath requires transpulmonary pressures >40 cm H2O.
Subsequent breaths require progressively less pressure as lung volume increases.

19. Cardiopulmonary Events at Birth (cont.)
Air in lung increases PO2 and pH, while PCO2 decreases, which results in:
Pulmonary vasodilation and decreased PVR
Ductus arteriosus constriction/closure
Increased pulmonary blood flow
At the same time, placenta removal results in:
Sudden increase in SVR
Net results:
LAP > RAP, so foramen ovale closes
Transition to extrauterine circulation complete
See Figure 8-9. Persistance of fetal circulation can lead to cardiopulmonary failure

22. Postnatal Upper Airway
Head flexion can cause airway obstruction.
Factors contributing to airway obstruction
Tongue is relatively larger compared with adults.
Nasal passages are relatively smaller.
Most infants nose breathe exclusively.
At 4 to 5 months, most infants can breathe orally.
Infections or Intubations can cause obstruction at the cricoid cartilage (narrowest point) or the epiglottis, which is relatively longer and less flexible than that of an adult.

24. Vascular Development Basic structure is in place at birth.
Subsequent vascular growth involves increased smooth muscle growth and increased density of arterioles and capillaries in distal regions.
Lungs are unique as blood from RV and LV provide flow to alveoli microcirculation.
Pulmonary circulation from RV
Bronchial circulation from LV
Provides greater stability and resistance against the impact of disease processes

25. Lymphatic and Nervous Development
Lymph nodes and vessels are located in connective tissues beside pulmonary structures
Provide fluid control and defense
Absorbed fluid travels to hilar lymph nodes
Nervous tissue development
Brainstem centers for automatic control
Phrenic and intercostal nerves form to carry motor signals to diaphragm and intercostal muscles
Autonomic fibers form for smooth muscle control

26. Chest Wall Development, Diaphragm and Lung Volume
Infant thorax is more compliant than that of an adult.
FRC is established by the equal and opposing forces of chest wall to expand against the lungs tendency to collapse.
The infant’s more compliant thorax results in lower FRC and TLC.
Predisposes infant to early airway closure, atelectasis, V/Q mismatch, and hypoxemia
Combated by ending expiration early, results in gas trapping, narrowing the glottis, back pressure
Patient will make a grunting sound . .
FRC = Functional Residual Capacity, TLC = Total Lung Capacity