Abstract
Surfactant is an interfacial material of respiratory conduits and its annexes. It modulates the surface tension and innate immune defense of the lung. The alveolar surface film stretches as the lung expands, raising the surface tension, then molecules are packed as the lung deflates, lowering the surface tension.
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Notes
- 1.
The surface tension can also be named superficial tension or interfacial tension whether the interface separates a liquid and gas or 2 liquids.
- 2.
Lung elasticity can be assessed by administration of saline solution in isolated lungs that removes the contribution of the superficial tension. For a given lung volume, a greater pressure is required to inflate the lung filled with air than lung filled with a saline solution. Pressure–volume loops of liquid-filled lungs are displaced leftward and upward (low transpulmonary pressure and high compliance) and display less hysteresis. Lung inflation with air measures both tissue and surface tension components. Furthermore, there is a much smaller difference between the air- and liquid-filled curves on deflation than on inflation.
- 3.
The Laplace law indicates that the pressure within a spherical structure with surface tension associated with a liquid–gas interface is inversely proportional to the radius of the sphere:
$$p\ =\ 2T/R$$(T: surface tension, R: sphere radius). At a constant surface tension, small hollow spheres generate greater pressures than large spheres. Small spherical tanks connected in parallel to large spherical reservoirs can therefore empty into large ones.
- 4.
The criterion for the wetting and non-wetting of solids by liquids is the value of the contact angle between the solid and the liquid. A liquid wets a solid when the contact angle ranges between 0 and 90 degrees (concave meniscus in capillarity experiments). When the contact angle is greater than 90 degrees, the liquid does not wet the solid (convex meniscus in capillarity experiments). Three interfaces exist when a liquid droplet contacts a solid or a liquid rises in a capillary tube: (1) the gas–solid (GS), (2) liquid–solid (LS), and (3) gas–liquid (GL) interface. Each interface is associated with a surface tension \(T_{GS/LS/GL}\). The contact angle α can be calculated from the following formula:
$$\cos \alpha = (T_{GS} - T_{LS})/T_{GL}.$$Partial or total wetting can be characterized by a spreading coefficient. When it is positive, the energy of the dry surface is greater than that of the wetted surface, and conversely when the spreading coefficient is negative. The droplet volume change can modify the contact angle without disturbing the equilibrium between the liquid droplet and the more or less rough solid wall. However, when the contact angle is greater than the expansion limit or smaller than the retraction limit, the droplet configuration changes.
- 5.
The motion of liquid in capillary tubes (rise or fall whether the liquid wets or not the solid wall of the tube of small radius (R < 5 mm) plunged into the liquid bath, according to the magnitude of capillarity forces, pressure and opposing gravity) is characterized by a meniscus (downward or upward meniscus whether the liquid wets or not the solid wall) at a height given by theJurin law:
$$H\ =\ 2T_{s}\cos \alpha /(R\rho g).$$Water barometers must then be based on tubes of inner diameter equal to 8 mm at least in order to avoid disturbed measurements.
- 6.
The cohesion forces are more important in liquids than in gases, with smaller molecule concentration. The higher the cohesion forces of a liquid, the stronger the surface tension, and the lower the liquid wetting.
- 7.
The total free energy of a system composed of 2 uniform fluids of densities ρ1 and ρ2, of volumes V 1 and V 2, and of specific free energy e 1 and e 2, separated by an interface of area A is given by:
$$e_{tot}\ =\ \rho _{1}V _{1}e_{1} + \rho _{2}V _{2}e_{2} + T_{s}A,$$where T s A represents the surface energy, T s being the surface tension that can hence be interpreted as free energy per unit area of the interface. The interface stretching work done by the surface tension in small reversible isothermal changes in the fluid system is equal to the gain in total free energy.
- 8.
The surface tension between air and water is equal to 73.10 − 3 N/m at 20 ∘ C (293 K) and to 68.10 − 3 N/m at 50 ∘ C (323 K).
- 9.
Surfactant components are packed by type-2 pneumocytes in lamellar bodies, although certain components such as SPa can also be secreted separately from lamellar bodies.
- 10.
Lamellar bodies are granules that contain SPa to SPc and lipids [1611].
- 11.
The ratio of phosphatidylcholine to sphyngomyelin varies from 2:1 in Eustachian tube surfactant to 67:1 in lung surfactant [1559]. In particular, dipalmitoyl phosphatidylcholine concentration is much higher in lung surfactant than in that of Eustachian tube surfactant.
- 12.
Protein SPa neck domain is involved in protein trimerization. Its globular C-terminus acts in lipid binding as well as formation and stabilization of curved membranes. Protein SPa exists in open or closed forms according to the medium composition. Calcium ions produce the closed structure of 6 trimers. The N-terminus of SPa protein is required for oligomerization as well as binding and aggregation of phospholipids [1614]. Its collagen-like domain is involved in SPa stability and oligomerization. It contributes to SPa shape and dimension.
- 13.
Saposins activate several lysosomal hydrolases involved in sphingolipid metabolism.
- 14.
The artificial monolayer is composed of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol in a molar ratio of 4:1 with 0.2 mol% SPb.
- 15.
Protein SPd is equivalent to conglutinin in blood circulation.
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Thiriet, M. (2013). Surfactant. In: Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems. Biomathematical and Biomechanical Modeling of the Circulatory and Ventilatory Systems, vol 5. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-5966-8_13
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