Pulmonary Surfactant

Respiratory system :
It is continuously synthesized and secreted by type II alveolar cells onto the alveolar surfaces, pulmonary surfactant is removed from these surfaces by alveolar macrophages and by type I and II alveolar cells. Its composition and continuous turnover allow it to serve 2 major functions. Not only does it reduce surface tension in the alveoli, but also it is thought to have some bactericidal effects, cleaning the alveolar surface and preventing bacterial invasion of the many capillaries in the septa.

The surfactant forms a thin 2-layer film over the entire alveolar surface. The film consists of an aqueous basal layer (bypopbase) composed mainly of protein, which is covered by a monomolecular film of phospholipid (mainly di palmitoyl lecithin) whose fatty acid tails extend into the lumen. By reducing surface tension, the surfactant helps prevent collapse of the alveoli during expiration. It thus eases breathing by decreasing the force required to reopen the alveoli during the next inspiration.

Because surfac tant secretion begins in the last weeks of fetal development, premature infants often suffer a condition called hyaline membrane disease, evidenced by respiratory distress (labored Ibreathing) caused by the lack of surfactant. Surfactant secretion can be induced by administering glucocorticoids, significantly improving the infant's condition and chances for survival.