Final answer:
Before birth, the fetal lungs and alveoli are filled with amniotic fluid, mucus, and surfactant, which are essential for lung development.
Step-by-step explanation:
Before birth, the alveoli in the fetal lungs are expanded and filled primarily with amniotic fluid. This fluid plays a critical role in lung development. Although the fetus "practices" breathing by inhaling this fluid, there is no air in the uterus to breathe. The fluid in the lungs also contains mucus and surfactant, which are essential for lung development. The placenta provides the fetus with all the oxygenated blood it needs, so there's no need for the fetal lungs to perform gas exchange at this stage. At birth, as the fetus is squeezed through the birth canal, much of the amniotic fluid is expelled from the lungs. Pulmonary surfactant, which is present in the lungs, helps reduce the surface tension of the alveoli and is critical for the inflation that occurs as the baby takes its first breath.