Final answer:
Before birth, the alveoli in the fetal lungs are filled with amniotic fluid. This fluid is expelled during birth, followed by the first inhalation of air which serves to inflate the lungs. The production and maturation of pulmonary surfactant is also a critical component of fetal lung development.
Step-by-step explanation:
Prior to birth, the alveoli in the fetal lungs are mainly filled with amniotic fluid. As the fetus passes through the birth canal, the thoracic cavity is compressed which expels much of this fluid. Any remaining fluid is rapidly absorbed by the body shortly after birth, and the first inhalation of air serves to inflate the lungs. The production of pulmonary surfactant, a substance critical for reducing the surface tension in the alveoli and thus facilitating breathing, begins around 20 weeks gestation, but doesn't generally create effective lung compliance until about the eighth month of pregnancy. Much of the development and maturation of the respiratory system occurs from week 24 until term, including the expansion of the surfaces for gas exchange (the respiratory membrane), and the growth and maturation of alveolar precursors into functional alveoli, a process that continues into early childhood.
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