[The significance of the veins for transcapillary fluid balance]

Various results from experimental as well as from theoretical studies challenge the classical concept of the Starling hypothesis of fluid balance between blood and tissue. Although most of these considerations are based on work with animal preparations, the following points appear to be applicable to human physiology: 1. The values of pressure as measured directly in venules, prohibit reabsorption of fluid from the interstitium. In addition, due to the increased permeability of the venular vessel wall to macromolecules, filtration of plasma proteins takes place at the venular end of the capillaries. 2. The overall permeability of the vessel walls to the proteins, is up to 20 times smaller than was previously assumed. Therefore, the total amount of filtered fluid is reduced to the volume range of the lymphatic transport. 3. The total amount of filtered proteins is brought back to the blood circulation via the lymphatic system. Thus, there is unidirectional slow movement of macromolecules with free fluid from the capillaries across the interstitial space toward the terminal lymphatics. However, exchange of small molecular substances such as water, gases, minerals and sugar by diffusion, is practically unlimited by the blood-tissue barrier. 4. As a practical example, the pressure values in the superficial veins of the human leg clearly exceed the limits for reabsorption of fluid from the tissue. The absence of edema in the normal human legs remains unexplained, if the lymphatic transport is not taken into account as an essential part of the system. Thus, the dynamic equilibrium of the fluid exchange is disturbed, either by increased transcapillary filtration (e. g. increase in venous pressure or microvascular permeability), by impaired lymphatic capacity or by a combination of these two factors.