Final answer:
Before birth, the fetal lungs' alveoli are filled with amniotic fluid, mucus, and surfactant, preparing for post-birth respiration. These substances are critical for lung development, particularly surfactant which is essential for reducing alveolar surface tension for the first breaths after birth.
Step-by-step explanation:
Before birth, the alveoli in the fetal lungs are expanded and filled with amniotic fluid, mucus, and surfactant. This environment is critical for lung development and prepares the infant for breathing air after birth. The presence of surfactant, which is produced increasingly after around week 24 of the pregnancy, is essential for reducing surface tension in the alveoli and making them more compliant, therefore allowing for easier inflation post-birth.
From weeks 24 to term, the respiratory system undergoes major growth and maturation, with the development of more alveolar precursors and production of larger amounts of pulmonary surfactant. It is important to note that the surfactant levels are not generally adequate until about the eighth month of pregnancy, which is necessary for effective lung compliance and the ability to breathe independently if born prematurely.
During birth, as the fetus is squeezed through the birth canal, this fluid is expelled from the alveoli, and the lungs quickly adapt to take over the role of the placenta, allowing the newborn to exchange gases and breathe independently.