THE EFFECTS OF TEMPERATURE AND OF TISSUE PRESSURE ON THE MOVEMENT OF FLUID THROUGH THE HUMAN CAPILLARY WALL

Abstract

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