Pulmonary surfactant is a surface active agent present in the alveoli of the lungs. This substance reduces the surface tension of water in the alveoli, a function that keeps the lungs from collapsing. Inadequate surfactant in the lungs can lead to respiratory distress, a condition that premature babies are especially prone to.
Why the Lungs Need Pulmonary Surfactant: Surface Tension
The lungs are made up of several tubes and tubules called bronchi and bronchioles which transport the atmospheric air in and out of the lungs. The ends of these tubes balloon out to form alveoli (singular alveolus) which are expandable, sac-like structures. The alveolus is the principal functioning unit of the lung. It is in these alveoli that the actual gas exchange takes place, oxygen getting absorbed into the blood stream and carbon dioxide being expelled.
Each alveolus is lined by a water surface which tends to contract. What does that mean? A drop of water contains many water molecules that have a strong inner force attracting the molecules to each other. This leads to contraction of the water surface or the surface of the water drop. Raindrops are a typical example for this contractile force exhibited by water, a force called surface tension.
Alveoli being lined by a layer of water are subjected to its strong surface tension. In effect, this would lead the alveoli to collapse, driving the air out of the alveoli and hence the lungs. This is where lung surfactant comes in.
Pulmonary Surfactant and Infant Respiratory Distress Syndrome
Surfactant is a mixture of compounds called phospholipids, mainly dipalmitoyl phosphatidyl choline, surfactant apoproteins and calcium ions. It is a surface active agent in water, working to reduce its surface tension. It is secreted by the alveolar epithelial cells (type II alveolar cells to be specific) and is spread on the water surface of the alveoli.
Every molecule of surfactant has a hydrophilic (attracted to water) and hydrophobic (water-repelling) parts. Surfactant works at the air-water interface in the alveoli with the hydrophilic portion immersed in water and the hydrophobic part in air, thus helping to reduce surface tension. It reduces the surface tension of the alveoli by four and a half times, preventing the lungs from collapsing. This also helps in reducing the energy or the effort required for the lungs to expand.
Alveoli with smaller diameter have a greater tendency to collapse, making the role of surfactant very important in them. This is typically seen in premature babies in whom the alveolar diameter is very small compared to adults. Surfactant is produced between the 6th and 7th months of pregnancy. This makes the lungs of premature babies born with minimal surfactant extremely prone to collapse resulting in respiratory distress syndrome. Commercially available surfactant can be instilled into the alveoli through positive pressure ventilation to treat this condition.
Pulmonary surfactant is a surface active substance that reduces the surface tension in the lungs and its deficiency causes infant respiratory distress syndrome.
Sources:
Guyton and Hall, “Textbook of Medical Physiology” (Saunders; 10th edition,2003)
Lakshmi Ananth - Lakshmi Ananth is a practising ENT, head and neck surgeon and has faculty position at a medical college. She is specially interested in ...
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