NONRESPIRATORY FUNCTIONS OF THE LUNG



In addition to its central role in gas exchange, the lung is active in both the metabolism and the degradation of many substances. Surfactant pro­duction by the alveolar Type II cell is an impor­tant metabolic function of the lungs. This phospholipid minimizes surface tension and thus confers stability on the alveoli and small airways, preventing atelectasis and decreasing the work of breathing. The failure of the immature lung to pro­duce sufficient surfactant leads to the respiratory distress syndrome (RDS) of the newborn. The lung is also involved in the biosynthesis of arachidonic acid into products of both the lipoxygenase and cyclo-oxygenase pathways. While a myriad of physiological functions have been ascribed to these agents, a clear relationship to pulmonary function is still lacking. Additionally, the lung is capable of removing or inactivating a large num­ber of biologically active substances, including serotonin, bradykinin, and prostaglandins. It is also the principal site of the conversion of angi­otensin I to angiotensin II.