Studies of factors influencing lung stability: biochemical changes of pulmonary surfactant and morphological changes of terminal air spaces in the developing rat.

Aging changes of the pulmonary surfactant were studied with regard to phospholipids and cholesterol and to the physical properties in the rat. Newborn rats had remarkably high content of lecithin compared to other age groups and the content was lowest at around 5 days after birth on the basis of wet weight of lungs. Calculated concentration per unit area of alveolar surface based on the body weight was highest in newborn rats and it fell down abruptly at 3 days after birth followed by a gradual decrease to adult level. The content of cholesterol was lowest and the lecithin:cholesterol ratio was highest in the newborn surfactant. The ratio was the same among rats at other ages. Surface activity examined by Wilhelmy balance was not different between newborn and adult with respect to minimum surface tension and stability index but surface spreading of surfactant was faster in neonatal surfactant than in the adult one. The size of the termnal air spaces was remarkably large at birth. It decreased gradually after birth with its minimum at around 17 days after birth and again it increased to adult level. From these observations it was concluded that high stability of excised lungs of normal newborn rats is brought about by these biochemical and morphological differences and these characteristics seem to facilitate the adaptation of lungs to normal breathing at birth.