[Transcapillary exchange of micro- and macro-molecules (author's transl)]

The permeability of the terminal vascular bed for micro- and macromolecules influences the hemorheological behaviour of the blood in various ways: 1. Water and water-soluble micromolecules leave the circulating blood at the arteriolar part of the microcirculation, thereby increasing the colloid osmotic pressure as well as the hematocrit downstream towards the capillaris and venules. 2. Water enters the blood from the tissue preferentially at the venules. This process may be disturbed easily by variations of the hydrostatic pressures with serious consequences for the streaming blood and for the tissue. 3. The permeability of macromolecules takes place mostly as the venous part of the capillaries. It depends on the molecular size and will be governed by the vessel wall itself. The coagulation system influences these processes. Together with the reabsorption of water from the tissue the protein movements through the capillary wall stabilize the hemorheological factors within the venules. The venules are the most vulnerable part of the circulation. 4. The movements of plasma proteins within the interstitial tissue are caused by concentration gradients. Finally the lymph collects the extravascular proteins. Within both compartments, th extravascular space and the lymph, various microrheological phenomena are to be expected.