Pulmonary surfactant

Image:Alveoli diagram.png Pulmonary surfactant is a surface-active agent formed by type II alveolar cells. The proteins and lipids that comprise surfactanthave both a hydrophilicregion and a hydrophobicregion, and so reduce the surface tensionat the air-liquid interface of alveoli. It does this by covering the surface of the liquid with the hydrophilic headgroups in the water and the hydrophobic tails facing towards the air.

Function

• To increase pulmonary compliance
• To prevent the lung collapsing into one large alveolus

In the lungs the situation is similar to that of an air bubble in water, as the alveoli are wet and surround a central air space. In this situation, the surface tension serves to try to make the bubble smaller (by decreasing the surface area of the interface), and this is counteracted by the pressure of the gas. Laplace?s equation expresses this equilibrium:

<math>P = \frac{2T}{r}</math>

Compliance

This causes the surface tension part of elastic recoil observed by Neergaard. If the lungs did not secrete surfactant, this surface tension would be much higher and it would not be able to inflate normally. The normal surface tension for water is 70 dynes/cm and in the lungs it is 25 dynes/cm.

Once released into the surface water layer, the surfactant forms a meshwork of tubular myelin. This can be broken down both by macrophages and reabsorbed into lamellar structures.

Pulmonary surfactant greatly reduces surface tension, increasing compliance allowing the lung to inflate much more easily, thereby eliminating the work of breathing. It reduces the pressure difference needed to allow the lung to inflate. The reduction in surface tension also reduces fluid accumulation in the alveolus as the surface tension draws fluid across the alveolar wall and when surfactant is not present, this force is higher.

Alveolar size regulation

As the alveoli increase in size, the surfactant becomes more spread out over the surface of the liquid. This increases surface tension effectively slowing the rate of increase of the alveoli. This also helps all alveoli in the lungs expand at the same rate, as one that increases more quickly will experience a large rise in surface tension slowing its rate of expansion. It also means the rate of shrinking is more regular as if one reduces in size more quickly the surface tension will reduce more so other alveoli can contract more easily than it.

Composition

Image:Alveolar type II cell.jpg

Lipids

Over 90% of the surfactant is lipids; around half of which is dipalmitoylphosphatidylcholine (DPPC). This is structurally similar to a phospholipidas it has 2 16-carbon saturated chains and a phosphategroup with quaternary amine group attached. Phosphatidylcholine molecules form ~85% of the lipid in surfactant and have saturated acyl chains. Phosphatidylglycerol forms about 11% of the lipids in surfactant, it has unsaturated fatty acid chains that fluidize the lipid monolayer at the interface. Neutral lipids and cholesterol are also present. The components for these lipids diffuse from the blood into type II alveolar cells.

Proteins

The remaining 10% of surfactant is comprised of proteins. Half of this is plasma proteinsbut the rest is formed by apoproteins. These are SP-A,B,C and D. SP-A and SP-D confer innate immunity as they have carbohydrate recognition domains that allow them to coat bacteria and viruses promoting phagocytosis by macrophages. SP-A is also thought to be involved in a negative feedback mechanism to control the production of surfactant. SP-B and C are hydrophobic and are membrane proteins that increase the rate that surfactant spreads over the surface. SP-B and SP-C are required for proper biophysical function of the lung.

The apoproteins are produced by the secretory pathway in type II cells. They undergo much post-translational modification, ending up in the lamellar bodies. These are concentric rings of lipid and protein, about 1μm in diameter. Pulmonary surfactant in secreted by exocytosis.

Diseases

Infant respiratory distress syndrome(IRDS) is caused by lack of surfactant, commonly suffered by premature babies born before 28-32 weeks of gestation.

Hyaline membrane disease is an older term for IRDS.

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Pulmonary+surfactant Wikipedia article Pulmonary surfactant.