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Acronym for: surface active agent
Synonym: Anti-atelectasis factor
German: Surfactant

1 Definition

Surfactant is a surface-active substance produced by the type II alveolar cells in the lung. It is secreted to the surface of the alveolar epithelium.

2 Biochemistry

Surfactant mainly consists of lipids (neutral lipids and phospholipids), proteins and Ca2+ ions.

2.1 Lipids

The largest proportion of surfactant are lipids with over 90%, of which 80% in turn are phosphatidylcholines. Around half of the lipid fraction consists of dipalmitoylphosphatidylcholine (DPPC). This phospholipid has two saturated palmitoyl residues with 16 C-atoms each, and a phosphate group with a quarternary alkyl ammonium group attached.

Phosphatidylglycerol (PG) contributes around 11% of lipids. It has unsaturated fatty acid residues which reduce the viscosity of the lipid layer formed by the surfactant. Furthermore, you can find neutral fats and cholesterol in the lipid fraction.

2.2 Proteins

The remaining 10% of surfactant are proteins. Half of it consists of plasma proteins, the other half of special surfactant proteins (SP). They are also called surfacant-associated proteins. They comprise:

  • SP-A (SFTPA1),
  • SP-B (SFTPB),
  • SP-C (SFTPC) and
  • SP-D (SFTPD).

The function of SP-A and SP-D is immunological. They possess domains for the detection of carbohydrates. These enable them to interact with the surface antigens of bacteria and viruses. They make them detectable for macrophages that can phagocytose the germs. SP-A also serves the regulation of surfactant production via a negative feedback mechanism.

SP-B and SP-C are hydrophobic membrane proteins which accelerate the spread of the surfactant on the alveolar surface. They are essential for normal pulmonary function.

3 Physiology

Without surfactant, the alveoles, which are constantly covered by a liquid film, would collapse especially during expiration due to the high surface tension.

Respiration would be made difficult many times over, or it would be even impossible. Surfactant as emulsifying, surface-active substance reduces surface tension and keeps te alveoles open this way. This allows for an effective gas exchange.

The production of surfactant is reduced in smokers, during physical work, the production is increased.

4 Clinical presentation

Surfactant is just produced from the 24th week of pregnancy by the unborn child. In premature births, especially until the 34th week, there is a more or less marked surfactant deficiency, which can lead to neonatal respiratory distress syndrome.

You can treat this by administrating recombinant surfactant or surfactant gained from animal material.

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