Surfactant - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 790

tension. This augmentation is balanced by the alveolus size elevation. Conversely,
rising concentration of surfactant balances the effect of reduction in alveolar size
during exhalation.
The surfactant is thus very important because: (1) it promotes alveolar stability,
alveoli of varying radii coexisting without ventilation between them, and (2) it
reduces the transpulmonary pressure. Furthermore, the surfactant hinders fluid
exudation from the alveolar septa within the alveolar space. The surface tension, the
tissue osmotic pressure difference in the alveolar septum, and the capillary blood
pressure work to move fluid out of the alveolar capillaries. The capillary osmotic
pressure opposes this force combination. A favorable balance can be maintained by
a reduction in the surface tension.
The absence of surfactant can induce atelectasis (lung collapse). A reduction
in surfactant can occur during prolonged inhalation of 100% oxygen. Premature
newborns often have a reduction in or lack of surfactant (respiratory distress
syndrome).

13.3 Surfactant Composition

Surfactant is a mixture of approximately 90% lipids, mostly phospholipids, and 10%
proteins, mostly surfactant proteins (SPa-SPd). Constituents mainly responsible for
the surface tension are phospholipids, organized in a thick monomolecular layer at
the air-liquid interface of the alveoli, and contiguous with the underlying layer.
The most abundant surfactant phospholipid is dipalmitoyl phosphatidylcholine
(DPPC) [ 1606 , 1607 ].
Lung stability at low surface tension requires that the material of the monolayer
must sustain surface tension (up to 30 mN/m [ 1608 ]) over a long time. A monolayer
of dipalmitoyl phosphatidylcholine can bear such a loading, but the surfactant
monolayer must undergo DPPC enrichment during breathing. This adaptive process
is done via exclusion of non-DPPC molecules and insertion of new DPPCs.
However, surfactant replacement therapy requires both lipids and proteins [ 1609 ].

13.3.1 Surfactant Proteins

Surfactant proteins are either hydrophobic or -philic. They participate in: (1) inter-
facial layer formation; (2) layer stabilization during deflation; (3) reformation of
surface film on expansion; and (4) monolayer turnover and recycling [ 1610 ]. Cyclic
compression and traction of the liquid interface indeed induces a continuous loss of
material from the interface, leading to cellular uptake and reprocessing of surfactant
lipids and proteins, followed by very quick transfer to the liquid film.