[Permeability of the air-blood barrier to macromolecules]

Serum macromolecules such as albumin or type G immunoglobulins are found in the film lining the alveolus, by virtue of techniques of broncho-alveolar lavage as well as demonstration by immunocytochemistry. Passage of such molecules may be studied experimentally by the use of tracers. Two barriers are found: a first at the level of the vascular endothelium which is relatively permeable and a second which is relatively impermeable at the site of the alveolar epithelium. Rupture of this barrier during non-hemodynamic oedema is responsible for massive passage of serum proteins to the alveolus, in particular of fibrinogen.