NONRESPIRATORY FUNCTIONS OF THE LUNG
The unique in-series position of the pulmonary capillaries within the circulation allows them to act as a filter for debris in the bloodstream. The lungs’ high content of heparin and plasminogen activator facilitates the breakdown of entrapped fibrin debris. Although pulmonary capillaries have an average diameter of 7 μm, larger particles have been shown to pass through to the left heart.
The role of the pulmonary circulation as a reservoir for the systemic circulation was discussed above.
The lungs are metabolically very active organs. In addition to surfactant synthesis, pneumocytes account for a major portion of extrahepatic mixed-function oxidation. Neutrophils and macrophages in the lung produce O2-derived free radicals in response to infection. The pulmonary endothelium metabo-lizes a variety of vasoactive compounds, including norepinephrine, serotonin, bradykinin, and a variety of prostaglandins and leukotrienes. Histamine and epinephrine are generally not metabolized in the lungs; in fact the lungs can be a major site of hista-mine synthesis and release during allergic reactions.The lungs are also responsible for convert-ing angiotensin I to its physiologically active form, angiotensin II. The enzyme responsible, angioten-sin-converting enzyme, is bound on the surface of the pulmonary endothelium.
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