© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

9.4.1.3 Radial Distribution of Blood Elements

At low shear rates, red cells aggregate into rouleaux and migrate inward, forming a network of aggregates in the core of the tube. Individual rouleaux may incorporate 10-20 red cells, or more, creating by far the largest cellular elements normally present in the blood. At the highest shear rates, the rouleaux break up entirely into single red cells, and the red cells then distribute themselves more uniformly in the radial direction.

The radial distribution of white cells is also a function of flow conditions.^{1325,1333-1336} At low shear rates, under conditions allowing red cell aggregation, the white cells are displaced to the periphery of the flow by the much larger red cell rouleaux. At the highest shear rates, white cell concentration is highest along the tube axis, displacing some red cells, since most white cells are larger than individual red cells.

The radial distribution of the local platelet concentration during blood flow in tubes has also been investigated.^1337,1338 In plasma containing only platelets, the platelet distribution is radially uniform. However, in whole blood flow under all shear conditions, the platelet concentration is highest near the vessel wall. In arterioles, the platelet number density is about two times higher near the wall than in the center of the vessel.¹³⁴² Platelets are much smaller than either red cells or white cells, thus tend to be crowded out of the center whenever red cells or white cells are present.

These experimental observations are consistent with the general principle that during blood flow in a vessel, the largest particles, or "flow units," move toward the axial region, leaving the smallest particles more concentrated at the periphery.¹³³² In most cases, bloodborne medical nanorobots will be ~2 microns in diameter or smaller. As such, they will normally constitute the smallest particles in the bloodstream. More than any other blood element, free-floating nanorobots should tend to migrate nearest the blood vessel walls, although at high shear rates the radial diffusivity is significantly increased due to local fluid motions generated by red cell rotations (Section 3.2.2). Small molecules and complexes such as lipoproteins are less subject to erythrocyte-induced diffusivity enhancement, hence may migrate toward the walls.

Last updated on 21 February 2003