Lymphatic capillaries of the pig lung: TEM and SEM observations.

Pulmonary lymphatic vessels extend within the connective tissue sheets surrounding airways and blood vessels. Frequently in this location they also border the lobular parenchyma, but not lymphatic vessels have been found within intralobular compartments between blood capillaries and alveoli. The presence and distribution of lymphatic vessels in pulmonary tissue are consistent with an important role for the lymphatic system in the clearance of interstitial fluids in the lung. Pulmonary lymphatic channels have structural characteristics of initial lymphatics; their walls are formed only by an endothelial layer, and no muscular cells are present. A network of anchoring filaments and collagen and elastic fibers surrounds the vessel walls. Because the lung is a mobile organ the tissue undergoes compression and distension during respiratory phases. These modifications could have a role in the mechanisms for lymph formation and flow.