Surfactant is a complex naturally occurring substance made of six lipids (fats) and four proteins that is produced in the lungs. It can also be manufactured synthetically.
Surfactant reduces the surface tension of fluid in the lungs and helps make the small air sacs in the lungs (alveoli) more stable. This keeps them from collapsing when an individual exhales. In preparation for breathing air, fetuses begin making surfactant while still in the womb. Babies that are born very prematurely often lack adequate surfactant and must receive surfactant replacement therapy immediately after birth in order to breathe.
Babies are considered premature if they are born before 37 weeks gestation. Fetuses begin to produce surfactant between weeks 24 and 28. By about 35 weeks, most babies have enough naturally produced surfactant to keep the alveoli from collapsing. Babies born before 35 weeks, especially those born very prematurely (before 30 weeks), are likely to need surfactant replacement therapy. Over half the babies born before 28 weeks gestation need this treatment, while about one-third born between 32 and 36 weeks need supplemental surfactant. Some very premature infants may also need to be placed on a mechanical ventilator.
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.
The administration of surfactant is often a neonatal emergency. The only way to prevent the need for surfactant replacement therapy is to prevent a premature birth. Mothers who are at known high risk to deliver prematurely are given drugs called corticosteroids toward the end of the pregnancy that stimulate the lungs of the fetus to mature and begin producing surfactant sooner. This helps reduce the need for surfactant replacement therapy. Although babies of all races may be born prematurely, prematurity is more common if the mother is diabetic, is carrying multiple fetuses, or has delivered a previous premature baby. The decision to use surfactant replacement therapy is based on the condition of the baby, its blood oxygen level, and degree of respiratory distress.
Babies receiving surfactant therapy are normally cared for by a neonatologist, a pediatrician that specializes in newborn care. Premature newborns often have other health problems in addition to RDS. Aftercare varies depending on their other health risks.
Delivery of surfactant requires inserting a breathing tube into the baby's lungs. Complications of this therapy include air leaking into the area between the chest wall and the lungs and air leaking into the sac around the heart. Some infants also develop chronic lung disease.
Normally surfactant replacement therapy keeps the infant alive until the lungs start producing their own surfactant.
Surfactant replacement therapy is very effective if begun within six hours after birth. When it fails, death may result.
Doctors Lounge, The. "Chronically Ventilated Premature Infants Need Continued Surfactant," 15 November 2004 [cited 16 February 2005]. 〈http://www.thedoctorslounge.net/pedlounge/articles/surfactant〉.
Hyaline Membrane Disease/Respiratory Distress Syndrome. Lucile Packard Children's Hospital at Stanford. 2001–2005 [cited 16 February 2005]. 〈http://www.lpch.org/DiseaseHealthInfo/HealthLibrary/hrnewborn/hmd.html〉.
Pramanik, Arun. Respiratory Distress Syndrome, 2 July 2002 [cited 16 February 2005]. 〈http://www.emedicine.com/ped/topic1993.htm〉.
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Alveolus (plural alveoli)— The terminal air sacs of the lungs where gas exchange occurs.
Hyaline membrane— A thin layer of cells that line the lung.
Surface tension— The attraction of molecules in a fluid for each other.
The most important site of surfactant is the lining of the alveoli of the lungs. Here it reduces the force needed to inflate the lungs and allows comfortable, quiet breathing. If you compare blowing up a bubble of a soap film with a party balloon, much more force is needed for the latter. This is because the molecules of the balloon stick together far more tightly than do those of soap solution; they are said to have a higher surface tension. In the 1920s it was shown that something in the alveoli must be reducing the surface tension of of the lining liquid, and this was subsequently shown to be surfactant. It is a mixture of fatty substances linked to proteins, the main ingredient being dipalmitoyl lecithin. It is made in one of the types of cell in the alveolar walls (type II cells), where it can be seen under the electron microscope as onion-like granules. Released into the airspace it spreads out and lines the alveolar surface.
In fetal life, surfactant first appears at about 20 weeks' gestation, and is being fully secreted by 30 weeks, 10 weeks before birth normally takes place. If it is absent the lungs are not only immature, but they can only be inflated with pressures 5–10 times greater than normal. Even if the baby can achieve this, it will rapidly lead to exhaustion. The condition is called Respiratory Distress Syndrome of the Infant (RDSI). Between 20 and 30 weeks' gestation more and more surfactant appears and the premature baby is progressively better able to overcome the defect in its lungs if born during this period. Surfactant production can be encouraged by giving the mother steroids (e.g. cortisol) before delivery, but nowadays these are combined with attempts to put surfactant directly into the infant's lungs. This was first attempted in 1964, but it was twenty to thirty years before the treatment became widespread and successful for premature babies. Either surfactant extracted from animal lungs or a synthetic version is used, and it can be administered directly into the airways or as an aerosol.
Adults can suffer a rather similar condition to RDSI, called ARDS (A=adult). With major traumatic injuries, or in some cases of severe septic shock or tissue destruction, the lining of the alveoli is damaged and the surfactant is ineffective. This leads to serious respiratory difficulties, which can be treated by surfactant replacement.
Surfactants are found in many other sites in the body, as well as in the lungs. For example, in the stomach surfactants may act as a barrier on the surface of the mucosa, which may explain in part why our stomachs are not digested by their own gastric juice. In the airways surfactants probably act as lubricants, allowing mucus and other materials to be cleared easily from the lungs by coughing or by ciliary transport.
See also antenatal development; breathing; infancy; lungs.
sur·fac·tant / sərˈfaktənt/ • n. a substance that tends to reduce the surface tension of a liquid in which it is dissolved.