Using the lung as a metabolic factor for secretion of therapeutic proteins into the circulation. Vector administration to the lung might target the conducting airway epithelium, the alveolar epithelium, or both. Secretion of protein products from the basolateral membrane of epithelial cells must diffuse through the interstitium (composed of fibroblasts and connective tissue) and across vascular endothelial cells, to be delivered into the blood (as marked by arrows). The alveolar epithelia of the lung, which contain a prominent population of Type I pneumocytes, are in close proximity to the lung capillary network, making them ideal gene therapy targets for systemic delivery of proteins.