RDS occurs when there is not enough surfactant in the lungs. Surfactant is a liquid made by the lungs that helps keep the airways (alveoli) open. This liquid makes it possible for babies to breathe in air after delivery without the lung spaces collapsing. An unborn baby starts to make surfactant at about 26 weeks of pregnancy. If a baby is premature (born before 37 weeks of pregnancy), they may not have made enough surfactant yet.
When there is not enough surfactant, the tiny alveoli collapse with each breath. As the alveoli collapse, damaged cells collect in the airways. They further affect breathing. The baby has to work harder and harder to breathe trying to reinflate the collapsed airways.
As the baby's lung function gets worse, the baby takes in less oxygen. More carbon dioxide builds up in the blood. This can lead to increased acid in the blood (acidosis). This condition can affect other body organs. Without treatment, the baby becomes exhausted trying to breathe and over time gives up. A ventilator must do the work of breathing instead.