Surface Lining of Lung Alveoli

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Abstract
The lining of the alveoli of the mammalian lung consists of an insoluble film of low surface tension formed from a saline-dispersible lipoprotein complex rich in phospholipids and of high molecular weight. The stability of the normal alveolus is due to high internal pressure, large radius, low surface tension and high surface elastance. The infants lung with respiratory distress syndrome is characterized by a high minimum surface tension and low surface elastance. Normally, the lining film and lining complex act so as to enable the lung to retain air at low inflation pressures by lowering surface tension and providing rapid variation of tension with area, and by preventing transudation. Evidence that the "hyaline membrane" is formed by transudation from blood is given by the demonstration of fibrin therein and from the findings that the prominence of the membrane increases with the time of survival of the infant. The mature fetal lung has a lining film. The early fetal lung is lined with cuboidal epithelium. The lining film appears at the time the cuboidal epithelium begins to desquamate. Most pathological conditions show no evidence that capacity to form a lining was reduced. Less stable bubbles were found upon intratracheal administration of surface-active substances such as lecithin and in some inflammatory conditions. In the condition called respiratory distress syndrome, the lung is unable to form a proper lining film causing the lungs to collapse after each inspiratory effort. Reptile and amphibian lungs have a lining film not identical with that of birds and mammals but still capable of lowering the surface tension to a low value.

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