Surfactant is a substances produced by T2 pneumocytes in the lung
It is composed of
Fast turnover, \(T_{1/2} \approx\)12 hours
Surface tension forces
In alvoli there is a air-fluid interface. Water molecules attract each other, which tends to collapse the alveolus. The collapsing pressure is given by the young-laplace equation for a sphere: $$\Delta P = \frac{2 T}{r}$$
Small alveoli would tend to collapse due to pressure diffrerence with large alveoli, and low intraalveolar pressure draws in fluid \(\to\) pulmonary oedema
Effect of surfactant
DPPC is amphipathic (hydrophilic glycerol lead, hydrophobic fatty acid tail). Sits at air-fluid interface \(\to\) steric hindrance \(\to\ \ \downarrow\)tension.
\(\downarrow\)Alveolar radius \(\to\ \ \uparrow\)[surfactant] \(\to\) larger effect, stabilizing small alveoli.
Therefore
Hysteresis
During inspiration, surfactant moves slowly from fluid phase \(\to\) interface; during expiration already at interface. This produces hysteresis (for any given pressure, lung volume is higher during expiration than inspiration).