surfactant

surfactant is a chemical that sounds like a detergent — which it is. If you could get enough you could try it in a dishwasher, although it would froth too much: the word is from ‘surf’ or sea-froth.

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.

John Widdicombe

See also antenatal development; breathing; infancy; lungs.