Surfactant - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 787

falls with lung deflation, maintains the alveoli in an inflation state, and helps them
to expand again rather than to collapse. The surfactant, thereby, prevents atelectasis.
Surfactant deficiency in respiratory distress syndrome was demonstrated from lung
extracts of babies in 1959 [ 1603 ]. Surface tension in compressed lung extracts of
babies with respiratory distress syndrome remains high. Surface tension increases
in expended lung extracts, but to much greater values compared to normal lung
extracts.

13.2 Alveolar Surfactant

Alveolar epithelium produces a liquid film over its surface in contact with air,
generating a fluid-air interface. Surfactant is made in type-2 alveolar cells. The
functional surfactant adsorbs very rapidly into the liquid interface to form a
tensoactive monolayer. After secretion, the surfactant causes less densely packed
arrangements in the structure of the air-liquid interface.
Surfactant is a material that contains phospholipids, predominantly dipalmitoyl
lecithin, and apoproteins. It is included in a fluid layer (thickness

70 nm).
The surfactant reduces surface tension throughout the major part of the lung
parenchyma, thereby contributing to lung compliance. It also stabilizes the alveoli.

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13.2.1 Formation of the Alveolar Surface Film

Surfactant is formed relatively late in fetal life. In utero, the fetal lung secretes
fluid into the alveolar spaces to promote lung development. Shortly before birth,
the fetal lung switches from fluid secretion to resorption to clear alveoli of excess
fluid and establish a proper air-blood barrier for gas exchange. After birth, gas
exchange depends on adequate regulation of the amount of fluid in the very thin
liquid layer (thickness 0.2 mm) that lines the alveolar epithelium. Premature

infants born without adequate amounts experience respiratory distress and can die
in the absence of clearance of the excess alveolar fluid and restoration of alveolar
fluid homeostasis to reinstate normal gas exchange. The half-life of the surfactant
is equal to about 18 h.
The formation of the alveolar surface film encompasses 3 successive steps:
(1) the transport stage during which surfactant aggregates move from the subphase
toward the interface; (2) the attachment stage during which surfactant bilayers
associate with the pre-existing monolayer; and (3) the transfer stage during which
the structure of the surfactant-associated phospholipid bilayers in the aqueous
subphase open in order to enable surfactant transfer to the interfacial monolayer.
Once secreted, surfactant forms tubular myelin , a lung-specific lipid transport
molecule for insertion into the air-liquid interface. Tubular myelin, an ordered array

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