Indirect Method for Measurement of Pressure in Blood Capillaries

Abstract

The capillary hydrostatic pressure of blood was measured indirectly. We used a microneedle to occlude exposed mesenteric capillaries in rats and frogs without trauma and then determined the motion of the trapped red blood cells that resulted from the absorption or filtration of water through the capillary wall. The trajectory of red cells was determined from enlarged timed photomicrographs that cover both sides of the occlusion. Gain or loss of water was measured by following the trajectory of two or more red cells to avoid errors due to injury and incomplete occlusion and extrapolating the motion to the instant of occlusion. Pressures were calculated from Starling's formula for fluid exchange. We assumed that the tissue parameters are an unknown constant in acute preparations; we used available values of filtration coefficients and colloidal osmotic pressures. The results show that 70 to 80% of the capillaries tested lost fluid, and therefore, their blood hydraulic pressure was higher than colloidal osmotic pressure. The pressures calculated in this way were similar to those measured by techniques requiring puncture of the capillary wall. Capillary arteriovenous pressure differences were approximately 4 mm Hg.