Pulmonary surfactant is a phospholipid-rich material crucial for proper lung function. Let’s delve into its composition, metabolism, and physiological role.
Composition
Pulmonary surfactant primarily consists of:
- Phospholipids:
- 80-85% Phosphatidylcholine
- 6-10% Phosphatidylglycerol (PG)
- 3-5% Phosphatidylethanolamine
- Proteins (10-15%):
- Surfactant-specific proteins (SP):
- SP-A
- SP-B
- SP-C
- Surfactant-specific proteins (SP):
- Neutral lipids (to a lesser extent)
Surfactant Metabolism
The Type II alveolar cells in the lungs are responsible for surfactant metabolism. Here’s a simplified breakdown of the process:
- Assemblage: Occurs in the endoplasmic reticulum, forming the backbone (glycerol) and head group (choline, ethanolamine).
- Protein Incorporation: Proteins are added in the Golgi apparatus for maturation.
- Storage and Secretion: Mature surfactant is stored in lamellar bodies and then secreted onto the lining of alveolar epithelial cells.
- Recycling and Degradation: Some secreted surfactant is reuptaken for recycling, while the rest is broken down by lysosomes. The components are then reused for reassembly.
Physiological Role of Surfactant
Surfactant plays a critical role in reducing surface tension within the alveoli, preventing them from collapsing.
- Opening Pressure: Without surfactant, the opening pressure of alveoli is very high (around 25 cmH2O). Surfactant lowers this pressure to approximately 15 cmH2O, facilitating easier breathing.
- Residual Volume: Surfactant ensures a residual volume of air remains in the alveoli during expiration, preventing complete collapse.
Laplace’s Law
Laplace’s Law explains the relationship between pressure, surface tension, and alveolar radius:
$$P={\frac{2T}{r}}$$
$$\overline{{\mathbf{r}}}$$
This law highlights that the opening pressure (P) is directly proportional to surface tension (T) and inversely proportional to the radius (r) of the alveolus. Surfactant reduces surface tension, thereby decreasing the pressure required to keep the alveoli open.
Surfactant Deficiency
- Hyaline Membrane Disease (HMD): A condition in newborns characterized by surfactant deficiency, leading to difficulty breathing and respiratory distress.
- COPD: In adults, conditions like Chronic Obstructive Pulmonary Disease (COPD) can also be associated with impaired surfactant function.
Key Points:
- Phosphatidylglycerol (PG) presence indicates lung maturity.
- Surfactant increases lung elasticity.
- Alveolar collapse is known as atelectasis.
