Permeability aspects of blood tissue exchange.

The exchange of substances between the blood and tissue compartments can be analyzed at both a structural and a functional level. Much remains to be learned concerning the participation of the several components of the capillary wall in the movement of substances to and from the bloodstream. Previous interpretations of the capillary wall as a biologic membrane with fixed structural features are probably not correct. It has been clearly shown that constituents such as the endothelial cells, basement membrane, and endocapillary lining change independently of one another. Especially striking is the tendency the endothelial surfaces have to separate from one another in response to physiologic mediators, despite the presence of supposedly "tight junctions." The contribution of the basement membrane to the permeability of small molecules and even plasma proteins remains to be demonstrated despite the provocative evidence provided by electron microscope studies of the passage of colloidal substances across capillaries and venules. On a functional level, a central role is taken by the opening and closure of precapillary sphincters as a mechanism for increasing or decreasing the surface area for exchange. The feedback tying this activity to parenchymal cell metabolism appears to be related to some substance in continuous flux responsive to reduced tissue clearance. Regional differences in permeability are present despite similar structural features, suggesting that local factors may affect the make-up of the barrier at a molecular level.