The interstitium, more precisely the extracellular matrix of interstitial connective tissue, is “supplied” with H2O by the blood vessels, and, in turn, it gives H2O to the initial lymph vessels. On both sides, a balance of forces must prevail that (can) move the water from one system to the other or distribute it to the systems. Within the extracellular matrix, H2O is only present in small amounts in free form. The larger part is reversibly bound to proteoglycans. Due to their sugar parts, these can bind large amounts of water. It is, therefore, easy to imagine that, on the one hand, changes in the amount of fluid present in the tissue lead to a change in the quantity and type of proteoglycans, and on the other hand, even slight changes in the number and size of proteoglycans can change the amount of interstitial fluid. In the extracellular matrix, there are pre-lymphatic channels, which in addition to a fluid shift also facilitate a shift of macromolecular substances and cells.