• To increase
pulmonary compliance. • To prevent
atelectasis (collapse of the alveoli or atriums) at the end of expiration. • To facilitate recruitment of collapsed airways. Alveoli can be compared to gas in water, as the alveoli are wet and surround a central air space. Pulmonary surfactant significantly reduces the work of breathing by minimizing alveolar surface tension, allowing alveoli to expand more easily during inspiration. The
surface tension acts at the air-water interface and tends to make the bubble smaller (by decreasing the surface area of the interface). The gas pressure (
P) needed to keep an equilibrium between the collapsing force of
surface tension (
γ) and the expanding force of gas in an alveolus of radius
r is expressed by the
Young–Laplace equation: : P = \frac{2\gamma}{r}
Compliance Compliance is the ability of
lungs and
thorax to expand. Lung compliance is defined as the volume change per unit of pressure change across the lung. Measurements of lung volume obtained during the controlled inflation/deflation of a normal lung show that the volumes obtained during deflation exceed those during inflation, at a given pressure. This difference in inflation and deflation volumes at a given pressure is called
hysteresis and is due to the air-water surface tension that occurs at the beginning of inflation. However, surfactant decreases the alveolar
surface tension, as seen in cases of premature infants with
infant respiratory distress syndrome. The normal surface tension for water is 70 dyn/cm (70 mN/m) and in the lungs, it is 25 dyn/cm (25 mN/m); however, at the end of the expiration, compressed surfactant phospholipid molecules decrease the surface tension to very low, near-zero levels. Pulmonary surfactant thus greatly reduces
surface tension, increasing compliance allowing the lung to inflate much more easily, thereby reducing the work of breathing. It reduces the pressure difference needed to allow the lung to inflate. The lung's compliance, and ventilation decrease when
lung tissue becomes diseased and
fibrotic.
Alveolar size regulation As the alveoli increase in size, the surfactant becomes more spread out over the surface of the liquid. This increases surface tension effectively slowing the rate of expansion of the alveoli. This also helps all alveoli in the lungs expand at the same rate, as one that expands more quickly will experience a large rise in surface tension slowing its rate of expansion. It also means the rate of shrinking is more regular as if one reduces in size more quickly the surface tension will reduce more, so other alveoli can contract more easily than it can. Surfactant reduces surface tension more readily when the alveoli are smaller because the surfactant is more concentrated.
Prevention of fluid accumulation and maintenance of dryness of airways Surface tension draws fluid from capillaries to the alveolar spaces. Surfactant reduces fluid accumulation and keeps the airways dry by reducing surface tension.
Innate immunity Surfactant immune function is primarily attributed to two proteins:
SP-A and
SP-D. These proteins can bind to sugars on the surface of pathogens and thereby
opsonize them for uptake by phagocytes. It also regulates inflammatory responses and interacts with the adaptive immune response. Surfactant degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection. ==Composition==