Surface Tension in the Lungs
Surface tension is the tendency of molecules in a fluid to be pulled toward the center of the fluid. It is measured as an energy per unit area (J/m2) or as the force across a line (N/m). (Adamson, 1990, Measurement binder)
Surfactants are substances that reduce surface tension.
Why are surfactants important in the lungs?
- They prevent water droplets from blocking airways.
- High surface tension would tend to decrease the surface area of the lungs, thus making it harder to absorb air.
The surface tension of pure water is about 70 mN/m. With lung surfactant, it can drop lower than 2 mN/m. (Possmayer et al., 2001, Measurement online)
Components of lung surfactant:
- 35-40% dipalmitoyl phosphatidylcholine (DPPC), a phospholipid
- 30-45% other phospholipids
- 5-10% protein (SP-A, B, C, and D)
- cholesterols (neutral lipids) and trace amounts of other substances
(Possmayer et al., 1984, binder)
The exact proportion of surfactant to water in the lungs is not known. However, the concentration of DPPC is probably on the order of 10 mg/ml. (c.f. Putz et al., 1994, Measurement binder)
DPPC is the chemical that actually reduces surface tension.
Its cross-section looks something like this:
(Possmayer et al., 1984, binder; see also Weidemann and Vollhardt, 1996, Film Structure binder)
The polar segment is phosphorycholine, which is hydrophilic. The top segment consists of neutral fatty acids.
DPPC packs better than other phospholipids because the fatty acids are straight, not kinked. (Possmayer et al., 1984, binder)
Lung surfactant needs to maintain low surface tension under expansion and contraction. Pure DPPC adsorbs slowly and respreads badly. Proteins improve adsorption and respreading, especially the hydrophobic SP-B and SP-C. (Goerke, 1998, online)
What does surfactant look like in the lungs?
(Schürch et al., 1998, online)
On expansion, other chemicals help insert DPPC. These are "squeezed out" on contraction.
The concept of an aqueous subphase and DPPC-enriched layer is generally accepted. (The idea of a pure DPPC film at high compression has been challenged.) However, the monolayer aspect of this model seems to have been an extrapolation from standard methods of measuring surface tension, and lacks empirical support.
So what does the top layer really look like?
Looking at excised lung (from rabbits) under a microscope suggests that the top layer is several molecules thick. Sometimes it's in a layered pattern, and sometimes it's an amorphous blob. (Schürch et al., 1998, online)
Experiments with models of fluid:
Expand/ compress repeatedly in Langmuir-Wilhelmy trough, then remove and examine in microscope. Even pure DPPC will have areas of different phases (liquid-expanded or LE, liquid-condensed or LC, and solid or S). (Möhwald, 1990, Film Structure binder)
When proteins are added, one finds patches of pure DPPC interspersed with regions of mixed lipid and protein. On compression, these mixed regions rise and fold. Depending on the protein composition, they may buckle or form stacked, plateau-like regions. It has been suggested that these stacks are composed of bilayers. (Schürch et al., 1998, online)