A surface active substance (a naturally occurring substance), is a foamy, fatty liquid that acts similarly to the oils in the lungs. Without the surfactant, the small air sacs of the alveoli are less likely to open and more difficult to keep open because the air sacs will atrophy. Surfactants keep the small alveolar air sacs open. How surface active substances work A child’s lungs are responsible for delivering oxygen to the whole body. Oxygen is the “fuel” that keeps the body working, and the lungs also have the function of expelling carbon dioxide, the waste gas produced by the body’s normal work. This process is called “gas exchange”. The physiological and anatomical principles of gas exchange are explained below. Lower airway: The airways or bronchi deliver air to the lungs. During inspiration, air enters the tiny bronchioles of the lower airway. Eventually the air reaches the air sacs or alveoli. Alveoli: Alveoli are very small air sacs in the lungs that are loosely spongy and through which oxygen is transferred to the blood. Gas exchange (carbon dioxide removal): oxygen enters the bloodstream through very small blood vessels on the surface of the alveoli, which is why the large surface area of the alveoli is important: the larger the surface area, the easier it is for oxygen to enter the bloodstream. Surface-active substances promote gas exchange: surface-active substances are a foamy, fatty substance that clings to the inner surface of the alveoli. It reduces the alveolar surface tension and prevents alveolar constriction. The production of surfactant starts around 24 weeks after pregnancy, but it does not reach the appropriate amount until after 35 weeks. Inadequate gas exchange: If the fetus is born prematurely, the quality of the surface active substance in the lungs will be inadequate. Without surface active substances, surface tension causes the alveoli to contract. The lungs will then become stiff and will require more pressure to induce oxygen to be drawn into the lungs. Surface active substance and preterm birth: If there are signs of preterm birth, the mother may receive hormone injections to help the fetus produce surface active substance. Otherwise, the infant’s airway will need to be injected with a surfactant after birth to keep the airway open. Because surfactants usually appear around the 24th week of gestation, they are not produced in sufficient amounts until the 37th week. Therefore, preterm infants are susceptible to neonatal respiratory distress syndrome (NRDS) due to insufficient surface active substances, and exogenous surface active substances can be used if preterm infants lack surface active substances. The administration of surface active substances is delivered through an artificial airway inserted into the trachea or a tracheal tube, whether used immediately after birth in extremely immature preterm infants or later when respiratory disease develops. If the lung lesion is severe, the baby may need to be given multiple doses of the surface active substance.