Chapter 4: Cell Membrane Structure and FunctionIssues in Biology |
A full-term baby develops for 38 to 40 weeks in the protected environment of the mother’s uterus before birth. In contrast, premature babies enter the world after only 23 to 37 weeks of development, making their appearance from less than 1 to more than 4 months early. Each year about 250,000 premature infants are born in the United States. About 40,000 of these babies will develop a serious lung disorder known as hyaline membrane disease or respiratory distress syndrome (RDS). Those babies born most prematurely have the highest risk of developing RDS. For example, about 60% of babies born after less than 28 weeks of gestation will suffer from the condition, but only 5% of those born after 34 weeks of gestation will have this condition. In the 1960s nearly every infant with RDS died. However, progress in neonatal care has raised the survival rate so that chances of survival are quite high: About 94% of them will survive.
Respiratory distress syndrome results from the inability of the baby’s immature lungs to properly inflate. As a result, babies with RDS breathe rapidly but cannot take in sufficient amounts of oxygen to supply their tissues. Their tiny lips and nails turn blue from lack of oxygen. As a result of oxygen deprivation, additional complications arise, including stroke and bowel problems. See Nathan’s Page for information about what life is like for these tiny newborns. What makes it so difficult for the lungs to work properly in babies with RDS is the absence of a special slippery substance called surfactant. This material is secreted by lung cells and forms a coat on the interior of the tiny alveoli in the lungs where oxygen enters the bloodstream. The coating enables the alveoli to expand, contract, and then re-expand with each successive breath. Without the surfactant, the alveoli have a hard time refilling once air is exhaled.
Surfactant is composed almost entirely of phospholipids. In fact, more than 60% of surfactant is a single phospholipid known as phosphatidylcholine. Interestingly, phosphatidylcholine is also a major substance in a commonly used kitchen aid that prevents food from sticking to pots and pans. The cooking spray, Pam™, contains lecithin, another name for phosphatidylcholine. You may be able to imagine a treatment for RDS. Why not simply expose the infant’s lungs to a mixture of surfactant? In fact, in the late 1960s and 1970s that is exactly what physicians tried to do. Initial results were not very promising, but as surfactant preparations made from animal lungs became available, dramatic improvement in the survival of these tiny infants was observed. In 1990 an artificial surfactant was approved by the U.S. Food and Drug Administration. This is made of phosphatidylcholine mixed with two special types of alcohol that suspend the lipid and help it to spread across the surface of the lungs.
Delaying birth until the baby's lungs have matured is still preferred. A doctor can check if a baby has made surfactant by testing amniotic fluid collected by amniocentesis or sampled right after the mother's water breaks. If the baby has not yet made surfactant, the mother may be given betamethasone or dexamethasone to help the baby's lungs develop more quickly, as well as other drugs to try to stop labor and delay the birth. As you can see, phospholipids are not only important for membrane structure, but you owe each breath to their interesting properties—and they are saving thousands of premature infants each year from nearly certain death.