* To increase<unk>  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. 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: : 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,<unk>  decreases the alveolar surface tension, as seen in cases of premature infants suffering from 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<unk>  phospholipid molecules decrease the surface tension to very low, near-zero levels.<unk>  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<unk>  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.<unk>  reduces surface tension more readily when the alveoli are smaller because the<unk>  is more concentrated. Prevention of fluid accumulation and maintenance of dryness of airways Surface tension draws fluid from capillaries to the alveolar spaces.<unk>  reduces fluid accumulation and keeps the airways dry by reducing surface tension. Innate immunity<unk>  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.<unk>  degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection.
Pulmonary surfactant
Hyalobarrier
Bruch's membrane
Fluid compartments
Reticular membrane
Schneiderian membrane
Neurotrophic factors
Endobronchial valve