Surfactant - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 782

Chapter 13
Surfactant

“ ...; omne oleo tranquillari, et ob id urinantes ore spargere,
quoniam mitiget naturam asperam lucemque deportet;...
[...; the whole sea is made still with oil and therefore the divers
under the water scatter it with their mouths, because it allaies
the rough nature thereof, and carries a light with it;...] ”
(Caius Plinius Secundus [Pliny the Elder; 23-79], Naturalis
Historiae, Book II)

Surfactant is an interfacial material of respiratory conduits and its annexes. It modu-
lates 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.
The substances that lower the surface tension in small amounts are called
surface-active agents or surfactants. They reduce the energy required for the surface
production.

13.1 Surface Tension

The free surface of a liquid at rest is not necessarily flat and horizontal, especially in
the neighborhood of a vertical solid wall. The surface effects are indeed not always
negligible with respect to the surface (pressure) and body (gravity) forces. The free
surface of a liquid behaves like an elastic membrane bearing a uniform force per
unit length.
The surface tension is the force acting on the surface of a liquid that tends to
minimize the surface area. 1 The smaller the surface area of any interface between a

1 The surface tension can also be named superficial tension or interfacial tension whether the
interface separates a liquid and gas or 2 liquids.