Surfactant

For the proper functioning of the respiratory alveoli, a number of different required compounds must be secreted from the cells that comprise the structure of the alveoli. One such essential fluid is known as surfactant, and it is secreted primarily by the Type-II cells (2). This fluid is made from a variety of compounds of which 85% are phospholipids, 10 % are surface-associated proteins, and 5% are neutral lipids such as cholesterol (3). Where the phospholipids are concerned, the main contribution to this group is made by phosphatidylcholine as it accounts for roughly 60% of the phospholipids found in the surfactant (3).

The role of surfactant in the lung is considered two-fold. First, surfactant is a structural aid in that it increases lung compliance, reducing the amount of pressure required to inflate the lungs during inhalation, while also reducing the surface tension at the air-liquid interface (3). This reduction in the surface tension of the alveoli prevents their collapse, which is known as atelectasis (1). The second function of surfactant is to help with innate immune defense, protecting against possible invaders within the respiratory system (3).

Figure 6: The Molecule Phosphatidylcholine Found in Surfactant
(Retrieved from http://www.columbia.edu/cu/biology/courses/c2005/purves6/figure03-21.jpg)