Lymph and Plasma Proteins: Barriers to their Movement throughout the Extracellular Fluid

Abstract

During the development of the mammalian organism two vascular systems are formed in most tissues. The fluid they contain, blood plasma and lymph, together with that in the intervening connective tissuef comprises the extracellular fluid or the "milieu interieur" of Cl. Bernard (5). Each 'system is lined by endothelium which is supported in the larger vessels by layers of connective tissue and smooth muscle cells: in the smallest vessels, however, the endothelium usually has little support except a basement membrane, and even this may sometimes be absent. It is, therefore, mainly by exchanges through the thin walls of these small vessels that the cells of the body maintain their normal metabolism. Substances of small molecular size very rapidly exchange by diffusion; the macromolecules such as the proteins and lipoproteins move much more slowly from compartment to compartment throughout the extracellular phase.