_Pseudoglandular (5-17 weeks)
�No gas exchange
zones
�Lung resembles an
exocrine gland
_Canalicular (17-25 weeks)
�Terminal
bronchioles enlarge and branch 2-3 respiratory bronchioles then 3-6 alveolar ducts. Terminal
sacs begin to form
_VAscularized – caudal slower than cranial
_Terminal sac (25 weeks to 34 weeks) – blood flow
and surfactant
�Epithelium thins to
become type I like
�Capillaries grow in
�Blood air barrier
forms
_Type I and type II cells
�Surfactant
reduces surface tension allowing expansion.
_Alveolar period (late fetal to childhood)
�Surfactant
�Gas exchange
�Pulmonary vs systemic
circulation
�Alveoli
mature from age 3-8. Numbers increase from 50 million at birth and 300 million at age 8 (adult
number)