Fluid exchanges through capillary walls: a modification of the Starling hypothesis

In textbooks of physiology it is usually stated that fluids are extravasated from the arterial ends of capillaries, because, there, the outward transmural pressure of the blood exceeds its net inward osmotic pressure. At the venous ends, however, the position is reversed. The fluids are, therefore, reabsorbed. Inthis paper it is argued that this is not a true picture of what must really happen. For, having once become equal there is no reason why the pressures should ever cease to be so. Instead, fluids are absorbed at exactly that rate which just keeps pace with falling hydraulic pressure, as the blood proceeds along the capillary loop. Moreover, the forces are indeed shown to be equal at the extreme venous ends of the capillaries. It is further argued that, though this mechanism might operate anywhere in the horizontal position, it could not possibly work in the feet of a standing man. For here the hydraulic blood pressure everywhere greatly exceeds the osmotic pressure of plasma proteins. An addition to Starling's hypothesis is therefore suggested to adapt it to dependent tissues. An experiment by which the validity of this addition might be tested is described.