Final answer:
Fetal lungs are fluid-filled because they receive oxygenated blood from the placenta and do not need to breathe air. The first breath after birth initiates lung function for gas exchange. Pulmonary surfactant is essential for the lungs to inflate properly.
Step-by-step explanation:
The lungs of a fetus are filled with fluid during gestation and have very little metabolic activity because the fetus does not need to breathe air. Instead, the placenta supplies the fetus with oxygenated blood. However, the fetus does practice breathing movements by inhaling amniotic fluid which is crucial for lung development. At birth, several factors, including labor contractions which reduce oxygenated blood flow and elevating carbon dioxide levels in the blood, stimulate the newborn's first breath. The first breath, typically taken within 10 seconds after birth, inflates the lungs and enables the newborn to take over the task of the placenta in gas exchange.
Before birth, the lungs do not function in gas exchange; the placenta is responsible for getting oxygen to the fetus and removing carbon dioxide. This is why the fetus does not feel breathlessness despite their lungs being fluid-filled. Following birth, the baby's lungs must quickly adapt to the external environment to allow for breathing air. Pulmonary surfactant, which is produced in increased amounts late in gestation, plays a key role in reducing surface tension within the alveoli of the lungs to facilitate their inflation with air after birth.