Surfactant

 

Surfactant

Surfactant is a material or agent that is responsible for lowering the surface tension of the fluid of alveoli. Surfactant is produced by Type || pneumocyte in alveoli.

COMPOSITION OF SURFACTANT

• surfactant molecule is made up of 90% lipid & 10% protein. 

• Surfactant contain lipid which exists in a unique form, called dipalmitoylphosphatidylcholine (DPPC)

• Surfactant contains four proteins, called surfactant protein (SP)-A, SP-B, SP-C, and SP-D. 

•  SP-A and SP-D, are hydrophilic, and SP-B and SP-C are hydrophobic.

SYNTHESIS OF SURFACTANT

Surfactant is synthesized in the endoplasmic reticulum of alveolar type II pneumocytes. The surfactant molecules are stored in the cluster & these clusters are called lamellar bodies until they are secreted into liquid on the alveolar cells by exocytosis. The lamellar body makes a tubular arrangement, called tubular myelin, which is transported to the air-liquid interface to form a monolayer of surfactant film. The phospholipids are either internalized and degraded by macrophages or recycled back to the type II cells for reuse.

**Note that surfactant protein (SP)-A, SP-B, and SP-D are also synthesized in club cells in terminal bronchioles.**

HOW SURFACTANT REDUCE SURFACE TENSION

• The surfactant came in the air-fluid interface. Surfactant molecules have hydrophobic & hydrophilic parts, So the hydrophilic part is pulled by the water molecule and the Hydrophobic part floats in the fluid and pulls the surfactant molecule upward. 

• Surface tension is exerted due to the net downward attraction of water molecules, which present at the air-fluid interface but now surfactant molecules that present at the air-fluid interface are pulled with equal forces in the opposite direction.

• When the amount of surfactant is more than the amount of the water molecule than naturally surface tension is reduced.

FUNCTION OF THE SURFACTANT

1. Reduce surface tension

2. Surfactant make alveoli dry

3. Stabilize the alveoli size uniform during Inspiration & Expiration:  as the radius of the alveoli is decreased, the effect of surfactant increases as surfactant molecules get more concentrated in smaller alveoli compared to larger ones, thereby maximizing their impact on lowering surface tension.