- Inner surfaces of alveoli are kept moist by secretions of a watery fluid from cells in the alveolar wall.
- Surface tension could easily cause wet cells to stick together - difficult for alveoli to expand when breathing in.
To prevent this, the fluid contains surfactant which reduces surface tension and is detergent- like.
- Baby in the uterus doesn't use lungs for gaseous exchange - no need for surfactant to be secreted and so doesn't happen until fairly late in pregnancy.
- If baby is born prematurely…