What role do surfactants play in the lungs?

Surfactants in the lungs reduce surface tension, preventing the alveoli from collapsing during exhalation.

In more detail, surfactants are complex natural substances made of lipids and proteins that are secreted by the alveolar cells of the lungs. They play a crucial role in the respiratory system by reducing the surface tension of the fluid lining the alveoli, the tiny air sacs in the lungs where gas exchange occurs. This reduction in surface tension is vital in preventing the alveoli from collapsing or sticking together when air is exhaled.

The primary component of lung surfactants is a type of lipid called a phospholipid. Phospholipids have a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. When these phospholipids are in the watery environment of the alveoli, they arrange themselves so that their hydrophilic heads are in the water and their hydrophobic tails are in the air. This arrangement forms a thin layer that significantly reduces the surface tension of the water.

Without surfactants, the high surface tension of the water would cause the alveoli to collapse upon exhalation, making it much harder to breathe. This is because the work needed to re-expand the alveoli during inhalation would be significantly greater. In fact, premature babies often struggle with breathing because their lungs have not yet started producing enough surfactants.

In addition to preventing alveolar collapse, surfactants also contribute to the lung's immune defence. They contain proteins that can neutralise viruses and bacteria, helping to prevent infections. Therefore, surfactants play a dual role in the lungs, both as a physical mechanism to facilitate breathing and as a chemical defence against pathogens.