Surfactant: Easing Tension on Surface

Ever wondered why raindrops were spherical or what made those tiny water striders walk on water? It is surface tension that makes these things as they are. When a liquid such as water, is in contact with air, the surface of water that is in contact with air, contracts. This is attributable to the interaction among dipole (polar nature) water molecules. It may be thought of as an elastic sheet that is trying to shrink.

Imagine any water particle, that is not part of any surface: air or that of the container. This imaginary water particle will be subjected to forces from water particles from all directions. The resultant force acting on this particle will thus be zero. But it is not so for water particles lying on the surface.
As a result, cohesive forces among surrounding water particles predominate and surface tension ensues. When an insect (as shown in the picture) or even a small metal coin is put on its surface, it doesn't sink. Buoyancy (Archimedes' Principle) is not the contributing factor here, since the amount(weight) of the displaced water (liquid) is very small. You can do a small experiment to visualize surface tension. Cut a strip of paper into the following shape.
Place it gently on some water surface and let it float. Add a small drop of soapwater into the hollow of the arrow. See how the arrow darts ahead. Its because this soap water (or camphor) reduces the surface tension at the back, while the surface tension ahead remains unabated, causing this pull. Now add a bit of oil: it stops moving further. Oil increases surface tension. The above is one example where physics and biophysics play an important role in biology.

Our body has plenty of water molecules in it. Surface tension thus plays important role in the lungs, where water comes in contact with air. The interior of the alveoli is lined with fluids resembling water. Predictably, the inner side of the alveoli will also try to shrink due to the surface tension arising from the air fluid interface. This pull would be about 72 dynes/cm, had it been pure water. However, the alveolar fluid contains detergent like 'surfactant' molecules to prevent it from collapsing (shrinking). They reduce the tension to 5-30 dynes/cm. These surfactant molecules are secreted by type 2 alveolar cells in the lungs. Chemically, they are a combination of proteins and phospholipids; di palmitoyl phosphatidyl choline (DPPC) being the most important. These molecules are amphipathic, meaning that they have both hydrophilic and hydrophobic domains. The hydrophilic domains remain in touch with the fluid while the hydrophobic domains remain in contact with air. Thus while surface tension (T) is trying to collapse the alveoli, surfactant is trying to diminish it. The pressure on the alveoli can be mathematically given as P=2T/R, where P is the pressure that needs to be applied externally, in order to prevent any change in its volume, due to surface tension (T). R refers to the alveolar radius. The above equation known as the Laplace's equation clearly depicts that when the alveolar radius becomes narrow, as in expiration, P becomes great. This is where surfactant does its duty. In premature neonates, the amount of surfactant may be much lower than normal, owing to their immature lungs. Hence they have high risks of having collapse of the lung (atelectasis). Severe respiratory distress may occur and this condition is known as hyaline membrane disease (also called infant respiratory distress syndrome, IRDS, respiratory distress syndrome of the neonates). This condition needs to be treated urgently.