A fourteen-page paper, which appeared in The Journal ofPhysiology of 1896, contained an idea which has become one of the general principles of cardiovascular and renal physiology. The author was Ernest Henry Starling and the idea is generally known as Starling's hypothesis (Starling, 1896a). The paper described direct and indirect evidence for the movement of fluid from the tissues straight into the blood. In searching for a mechanism for this phenomenon, Starling suggested that, while microvascular walls were freely permeable to salts and small solutes such as glucose, proteins were severely impeded in their movements between the plasma and the tissues. He noted that the concentration of proteins in the plasma was considerably greater than that found in tissue fluids or lymph and proposed that this difference in concentration might be responsible for an osmotic pressure which opposed the greater hydrostatic pressure within the capillaries than in the tissue. Thus, a balance between the