The lungs perform several metabolic functions including gas exchange, conversion of angiotensin I to angiotensin II by angiotensin converting enzyme (ACE), inactivation of vasoactive substances, and protein synthesis.
Angiotensin converting enzyme is found on the surface of capillary endothelial cells in the lungs. The enzyme catalyzes the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor.
Many vasoactive substances are also inactivated during passage through the lungs. Bradykinin is inactivated by ACE. Serotonin is also inactivated in the lungs, not through degradation, but rather through uptake and storage. The lungs also contain enzymes that degrade prostaglandins E1, E2, and F2alpha, as well as norepinephrine.
The lungs are involved in immune function through production and secretion of IgA. The lungs also perform synthetic functions including formation of phospholipids, which form surfactant, as well as collagen and elastin protein synthesis.