Question

Surfactants in alveoli is secreted by
A) Dust cells
B) Type I epithelial cells
C) Type II epithelial cells
D) Kupffer cells

Answer 1

Alveoli is the structural and functional unit of the pulmonary organ that is lungs. They are the site for gaseous exchange. Pulmonary surfactant is a mixture of lipids and proteins. The main function is basically to reduce the surface tension at the surface of the lungs.

Complete answer:
When we talk about pulmonary surfactants, they are basically the surface-active lipoprotein complexes secreted by the cells in the alveolar wall. The proteins and lipids present on the surface help in making the surface of alveoli both hydrophobic and hydrophilic. On absorption to the air-water interface of alveoli with the hydrophobic part which is in the air and the other hydrophilic part facing into the air.

Let us first know about the major types of cells in the alveolar wall.
1. Type 1 or squamous alveolar cells - they form the structure of the alveolar wall.
2. Type 2 or great alveolar cells - they secrete pulmonary surfactants. Type 2 epithelial cells help in lowering down the surface tension of water. This allows the membrane to get separated, which helps to increase the capacity for the exchange of gases.

The main function of these cells are -
1. synthesis and secretion of surfactants.
2. xenobiotic metabolism
3. regeneration of alveolar epithelium
4. transepithelial movement of water

- Dust cells- These are the macrophages found in the pulmonary alveoli. It keeps the air surface clean by ingesting in debris, pollutants and other microorganisms entering the lungs.
- Kupffer cells- These are the phagocytic cells present in the liver. The main function of this cells is digestion of RBC.

Therefore the correct answer is option ‘C’.

Note: The cells present in the alveolar wall are essential for the exchange of gases and helps in maintaining the structure of the alveoli. As we know they are composed of proteins and lipid which are packed separately in the lamellar bodies. These bodies are the major components for the synthesis and metabolism of surfactants.