The purpose of this study was to investigate the pressure drop flow rate behavior of normal human blood flowing through small capillary tubes. A sensitive pressure transducer was used to measure the pressure gradient along an experimental tube when various red cell suspensions passed through glass and plastic capillary tubes. In particular, the effect of capillary tube diameter on the rheological properties of blood was observed for capillary tube diameters from8 to 221 microns.
For tubes of 221 microns and smaller, it was found that the hematocrit (volume fraction of red cells in a blood sample) of the blood flowing through a capillary tube is less than the hematocrit of the blood in the feed reservoir. The tube hematocrit decreases linearly with the logarithm of the tube diameter at constant feed reservoir hematocrit and for a given diameter tube, increases linearly with the feed reservoir hematocrit.
An equation was developed from data taken in an 811-micron ID tube that allows the shear stress-shear rate relation to be predicted from the tube diameter, tube hematocrit, and the temperature.
It was found that the rheological properties of blood can be accurately predicted from the equation developed if the average hematocrit inside the capillary tube is used as the correct hematocrit parameter. A surprising result found is that fluid properties can be predicted for blood flow through a 29-micron ID tube; in such a tube, the equation of motion may not be valid because the "continuous fluid" assumption is not valid. Blood flow data were taken at98.6°F as well as 73.5°F. Blood heated to 111°F and then cooled to96.6°F was also investigated.
Blood flow through 15- and 9-micron ID capillaries was investigated. Blood exhibits no yield stress in a 15-micron ID tube because rouleaux formation cannot restrict the flow as it does in larger tubes.
An increased yield stress was found in a 9-micron ID tube. The measured shear stress was only slightly larger (for a given U) than predicted by the continuum model in both the 15- and 9-micron ID capillary tubes.
