The Fahraeus effect

Abstract

Fahraeus reported in 1929 that when blood flows from a large diameter tube through a capillary tube, the average hematocrit of the capillary blood is less than that of the blood in the larger tube. We have investigated this Fahraeus effect by pumping human blood, and red cell-isotonic saline suspensions, from a large reservoir through capillaries 29–221 μ in diameter. The independent variables were the flow rate, the feed reservoir hematocrit ( H F), the capillary diameter, the stirring rate in the reservoir, and the continuous phase protein content. The measured dependent variables were the average hematocrit in the capillary tube ( H T), and the mixing-cup hematocrit of the blood flowing from the tube. Under steady flow conditions in the range of conditions studied, the tube relative hematocrit (defined as H R = H T H F ) was found to be independent of the blood flow rate, the protein content of the continuous phase of the red cell suspension, and the stirring rate in the reservoir. H R is a linear function of H F for a given capillary size; the straight line representing this function has a slope which increases, and an intercept which decreases as the tube diameter decreases. The mixing-cup hematocrit of the blood flowing from tubes 59μ in diameter or larger was always equal to the feed reservoir blood hematocrit. Unsteady flows result in H R values which differ from steady flow values. Our results agree with those of Fahraeus ( H F = 40%), but differ from those of Hochmuth and Davis for smaller capillaries. Besides providing quantitative data, our results support the contentions that flow conditions upstream from the capillary entrance are not responsible for the Fahraeus effect, and that the hematocrit in the capillary is a function of radial position. In 29-μ tubes, we have also observed an axial variation in hematocrit.