Lecture notes on pulmonary surfactant

Surfactant is a surface acting material or agent that is responsible for lowering the surface tension of a fluid. Surfactant that lines the epithelium of the alveoli in lungs known as pulmonary surfactant and it decreases the surface tension on the alveolar membrane.

Pulmonary surfactant is secreted by two types of cells:

1. Type II alveolar epithelial cells in the lungs, which are called surfactant secreting alveolar cells with microvilli on their alveolar surface.

2. Clara cells(bronchiolar exocrinecells.), which are situated in the bronchioles.

Surfactant is a lipoprotein complex formed by lipids especially phospholipids, proteins and ions.

1. Phospholipids: Phospholipids form about 75% of the surfactant. Major phospholipid present in

the surfactant is dipalmitoylphosphatidylcholine (DPPC).

2. Other lipids: Other lipid substances of surfactant are triglycerides and phosphatidylglycerol (PG).

3. Proteins: Proteins of the surfactant are called specific surfactant proteins. There are four main surfactant proteins, called SPA, SPB, SPC and SPD.SPA and SPD are hydrophilic, while SPB and SPC are hydrophobic. Surfactant proteins are vital components of surfactant and the surfactant becomes inactive in the absence of proteins.

4. Ions: Ions present in the surfactant are mostly calcium ions.

Functions of surfactant

1. Surfactant reduces the surface tension in the alveoli of lungs and prevents collapsing tendency of lungs.Phospholipid molecule in the surfactant has two portions. One portion of the molecule is hydrophilic. This portion dissolves in water and lines the alveoli. Other portion is hydrophobic and it is directed towards the alveolar air. This surface of the phospholipid along with other portion spreads over

the alveoli and reduces the surface tension. SPB and SPC play active role in this process.

2. Surfactant is responsible for stabilization of the alveoli, which is necessary to withstand the collapsing tendency.

3. It plays an important role in the inflation of lungs after birth. In fetus, the secretion of surfactant begins after the 3rd month.

4. Another important function of surfactant is its role in defense within the lungs against infection and inflammation. Hydrophilic proteins SPA and SPD destroy the bacteria and viruses by means of opsonization. These two proteins also control the formation of inflammatory mediators.