What is the role of surfactant in the lungs?

Surfactant in the lungs reduces surface tension, preventing the alveoli from collapsing during exhalation.

In more detail, surfactant is a complex mixture of lipids and proteins that is secreted by the type II alveolar cells in the lungs. Its primary role is to reduce the surface tension within the alveoli, the tiny air sacs in the lungs where gas exchange occurs. Surface tension is a force that causes liquid surfaces to shrink to the smallest possible area, and in the context of the lungs, it can cause the alveoli to collapse, particularly during exhalation. This would make breathing difficult and inefficient.

Surfactant counteracts this by reducing the surface tension, thus preventing the alveoli from collapsing and ensuring they remain open for efficient gas exchange. It achieves this by forming a thin layer over the inner surface of the alveoli. The lipids in the surfactant, primarily phospholipids, are amphipathic, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. The hydrophilic parts interact with the watery mucus in the alveoli, while the hydrophobic parts repel water, thereby reducing the surface tension.

In addition to this, surfactant also plays a role in the immune response within the lungs. The proteins in surfactant can bind to pathogens, such as bacteria and viruses, marking them for destruction by immune cells. This helps to protect the lungs from infection.

A lack of surfactant, or a dysfunction in its production, can lead to respiratory distress. This is particularly a concern in premature infants, whose lungs have not yet fully developed and may not produce enough surfactant. This condition, known as neonatal respiratory distress syndrome, can be life-threatening and often requires medical intervention to treat.

In summary, surfactant plays a crucial role in the lungs by reducing surface tension to prevent alveolar collapse, facilitating efficient gas exchange, and contributing to immune defence.