Surfactant

Overview :

The lungs consist of spongy tissue filled with air spaces called alveoli. In the alveoli, oxygen is taken up by the blood and carbon dioxide, a waste product of cellular metabolism, is released and exhaled. For efficient oxygen-carbon dioxide exchange to occur, the surface area of the alveoli must be as large as possible. Under normal conditions, when a person exhales, the alveoli would collapse into each other and form larger air sacs with less surface area. Surfactant prevents this collapse by reducing the surface tension of the fluids that line the lungs and helping to equalize the pressures between large and small air spaces.

Surface tension is a measure of the attraction molecules of a fluid have for each other. The attractive force pulls fluids into a shape with the smallest surface area. This is why a drop of water on a flat surface is rounded rather than flat. If the surface tension is lowered, the attraction among molecules of the fluid is decreased and the surface area of the fluid increases. For example, if a drop of detergent is added to a drop of water, the detergent reduces the surface tension and the drop of water flattens out.

In the lungs, surfactant reduces the surface tension and helps to maximize the surface area available for gas exchange. Without adequate surfactant, a baby works much harder to breathe, becomes exhausted, and does not get enough oxygen. Babies that do not have enough surfactant to breathe normally at birth are said to have infant respiratory distress syndrome (RDS) or hyaline membrane disease (HMD).

Babies with RDS are given replacement surfactant as soon as possible within the first six hours after birth. Manufactured surfactant is a white powder that is mixed with sterile water. It is given through a breathing tube (endotracheal tube) that is inserted in the baby's lungs. Multiple doses are usually required.

Surfactant replacement therapy continues until the baby's lungs have matured enough to make surfactant on their own. Some very premature babies are also put on mechanical respirators to help them breathe. Surfactant replacement therapy has reduced deaths due to respiratory distress by 50% since the early 1990s. This therapy is expensive, but it is normally covered by insurance.