Factors Controlling the Movement of Fluid Through the Human Capillary Wall *

Blood, the transporting medium of the body, is practically everywhere separated from the tissues, which it serves, by a single layer of endothelial cells forming the walls of the capillary network. During the past century the capillaries were believed to be inert, thin-walled tubes conveying blood to the tissues in whatever quantity the arteries might supply. The studies of Krogh4 and Lewis"4 have shown, however, that the capillary vessels are independently contractile, and that they are capable of responding individually in a very delicate manner to the circulatory needs of the immediately adjacent tissues. But even so, the blood is still separated from the tissues by the capillary wall, and the permeability of this single layer of endothelial cells will be intimately concerned with the rate and the manner of interchange of substances between the blood stream and the tissue spaces. For example, when water is absorbed from the intestinal tract, in order to enter the blood stream it must pass inward through the walls of the intestinal capillaries. From the blood the absorbed water may pass outward through the glomerular capillary wall or may find its way through the walls of the systemic capillaries into the general tissue spaces. In either case, the fluid must pass through the capillary wall again, this time, however, in the opposite direction. With this continual movement of fluid through the capillary wall, at one moment inward, at another outward, it is remarkable that the amount of tissue fluid remains approximately the same and that the circulating blood maintains a constant volume. To explain this equilibrium an attractive hypothesis was advanced by Starling"7 in 1896. It had been shown that the capillary endothelium is easily permeable to water and to solutions of most salts but is relatively impermeable to the proteins of blood plasma by reason of their colloidal dimensions. Starling emphasized the fact that the enormous osmotic pressure of the blood plasma, amounting to between six and seven atmospheres, is due almost entirely to dissolved salts. But since these salts pass easily through the capillary