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It is well established by experiments on normal human subjects that when venous pressure is elevated, fluid is filtered from the blood into the tissue spaces. This filtration has been identified by the increase in the volume of the congested limb (Mende, 1919;Carrier and Rehberg, 1923; Drury and Jones, 1927); and also by the concentration of protein in the blood which has passed through the congested limb (Rowe, 1916). There is no information, however, concerning the effect in normal human subjects of changing the other equally important factor, namely, the colloid osmotic pressure of the blood.The association of a low colloid osmotic pressure of the blood with certain forms of edema occurring as a result of disease in man (Krogh, 1922;Hagedorn, Rasmussen and Rehberg, 1922; Govaerts, 1924;Schade and Claussen, 1924; and others), or occurring with experimental hypoproteinemia in dogs (Leiter, 1928) suggests, but does not prove, that the movement of fluid through the capillary wall is related to the colloid osmotic pressure of the blood. The further finding that capillary pressures in the frog, certain rodents, and man (Landis, 1926(Landis, , 1930 are approximately equivalent to their different blood colloid osmotic pressures is also suggestive, but not conclusive, evidence that the Starling conception probably underlies the mechanism of fluid balance in the mammal.If fluid balance in man depends upon an approximate balance between capillary pressure and the colloid osmotic pressure of the blood two things must follow. In the first place, as mentioned by Krogh (1929), a relatively small elevation of venous pressure must suffice to cause fluid to accumulate in the tissue spaces, so that the erect human being is constantly near to edema. In the second place, with any given capillary pressure, a rise in the colloid osmotic pressure of the blood must be accompanied by a fall in the rate of filtration.

The various physical factors concerned in transporting fluid through the human capillary wall have received relatively little attention in spite of the value that such information might have in clarifying the complicated mechanism of fluid balance and the even more involved mechanism of edema formation. The present lack of information is to be ascribed, in all probability, to the difficulty of obtaining quantitative data concerning the filtration and absorption of small amounts of tissue fluid. At present the plethysmograph offers the only available means of detecting small changes in the volume of extravascular fluid.The ordinary plethysmograph was found by Krogh, Landis and Turner (1) to be quite unsuitable for measuring the volume of tissue fluid accumulating during short periods of slight or moderate venous congestion. Therefore, a so-called "pressure plethysmograph " was devised to exclude spontaneous variations in arm volume referable to changing vasomotor tone. Pressure was exerted on the surface of the segment of forearm within the plethysmograph in order to collapse the blood vessels before the final volume was measured. Under these conditions the state of contraction or dilatation of the blood vessels did not, within certain limits, interfere with the reasonably accurate measurement of changes in the volume of tissue fluid.It was found in normal subjects that fluid was filtered into the tissues of the forearm when the venous pressure exceeded 15 cm. water. Above an average venous pressure of 17 cm. water the rate of filtration was directly proportional to the increase in venous pressure. WVhen the subject stood motionless the colloid osmotic pressure of the blood rose and the rate of filtration produced by given grades of venous congestion fell. Thus changes in capillary pressure and in the colloid osmotic pressure of the blood influenced the movement of fluid through the capillary walls of normal human subjects in a manner conforming to the Starling hypothesis.If capillary blood pressure and the colloid osmotic pressure of the blood were the sole factors involved in fluid balance it would be difficult to explain how it is possible for the human being to avoid dependent 105

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