The American Journal of Physiology

Previous work on the vessels of the frog mesentery by the micro-injection method (Landis, 1926a) has indicated a balance between the average capillary blood pressure and the osmotie pressure of the plasma proteins. The fact that the average pressure in the arteriolar capillary is aboveand in the venous capillary is below-the osmotic pressure of the plasma colloids is regarded as evidence favoring the hypothesis of Starling (1896). According to this view fluid leaves the blood stream when capillary pressure rises above the osmotic pressure of the plasma proteins, to which the normal endothelium is impermeable; and enters the blood stream when the capillary pressure falls below that level. This paper deals with further experiments designed to study the permeability of the capillary wall in a quantitative manner, with control of capillary pressure. The experiments fall into two groups. In the first the movement of fluid through the capillary wall is identified indirectly by injecting solutions of certain easily visible dyes; in the second the rate of fluid passage inward or outward through the endothelium is measured directly by the respective dilution or concentration of the corpuscles in the capillary. Both methods provide further evidence of a more direct character favoring the view that fluid movement depends upon the level of capillary pressure in relation to the effective osmotic pressure of the plasma colloids. I. THEPASSAGEOF DYESOLUTIONSINRELATIONTOCAPILLARY PRESSURE AND DILATATION. The rate of movement of dye solutions through the 217