Roe Wells

Fluid Drop-Like Transition of Erythrocytes under Shear

Red cells dispersed in a continuous medium of high viscosity possess the flow properties of fluid drops. The cells at rest are biconcave, while under shear they become progressively deformed into prolate ellipsoids, their long axis aligned parallel to the flow direction. The red cell membrane rotates around the hemoglobin like a tread of a tank. At high rates of shear this mechanism greatly reduces viscosity at all hematocrit values.

Influence of Deformability of Human Red Cells upon Blood Viscosity

The viscosity of blood at high rates of shear is unusually low compared to other suspensions of similar concentration. The underlying mechanisms were studied by rotational viscometry, red cell filtration, viscometry of packed cells and direct microscopic observation of red cells under flow in a transparent cone plate viscometer. Deformability of red cells was altered osmotically or abolished by aldehyde fixation. The normal red cells under isosmotic conditions passed easily through filter pores (5 to 14 μ diameter). After osmotic crenation, deformability of cells in pore flow was reduced. Normal cells were deformed into a variety of shapes at high rates of shear, while crenated cells tumbled undeformed. Suspensions of these normal cells showed more pronounced shear thinning (reduction of viscosity with increasing shear rate) than suspensions of crenated cells. Suspensions of rigid cells showed greatly increased viscosity and a shear thickening as a function of shear rate and shear time. The physiological deformability is of critical importance to blood flow at high rates of shear. This is possible through a fluid transition of the erythrocyte caused by a rotation of the membrane with and around the cell contents. This phenomenon is the prime cause of the progressive reduction in viscosity with increasing shear.

Non-Newtonian Rheology of Human Blood - Effect of Fibrinogen Deduced by "Subtraction"

A study of the rheological properties of human blood, from donors in normal health, was carried out by means of a coaxial cylinder viscometer designed to measure very small levels of stress under conditions of “creeping” flow. It was found that under these conditions of measurement the rheological properties could be conveniently presented by plotting the square root of shear stress against square root of shear rate. For normal blood, a nearly linear relation is found on such a plot, and the intercept on the stress axis at zero shear rate represents the square root of yield stress, separate determination of which is made by other means. Similar plots for (i) defibrinated blood and (ii) suspensions of red cells in isotonic saline solution reveal no yield stress. Thus it is concluded that fibrinogen is essential for the existence of yield stress in human blood. Furthermore, the approximate linearity, for normal blood, of the square root of shear stress with square root of shear rate, and the yield stress intercept, are of great interest inasmuch as mathematically identical relations ensue according to an equation developed by Casson based on a physical model in which the elementary particles of a suspension are capable of reversible association into rod-like structures, the length of which is controlled by the shear rate. It is of interest to consider the Casson model in the light of rouleaux formation and the relation of fibrinogen to rouleaux formation.

Shear Rate Dependence of the Viscosity of Whole Blood and Plasma

The analysis of the shear stress/shear rate relationship, and thus the viscosity/shear rate relation, of blood and plasma shows that (i) freshly drawn whole blood has a large shear rate dependence on viscosity (viscosity falls as shear rate increases), and (ii) the shear rate dependence of viscosity of whole blood, or plasma, that has not been treated to prevent clotting is substantially greater than that of whole blood or plasma treated with anticoagulants. The influence of this phenomenon upon the fluid mechanics of the microcirculation is commented upon.

Effects of hemorrhagic shock on the microvasculature of skeletal muscle

Abstract The relationship between arteriolar and venular dimensions and the progressive failure of the homeostatic mechanisms leading to irreversibility in hemorrhagic shock was evaluated in mammalian skeletal muscle (rat cremaster). The small distribution arterioles (diameter = 17 μm) were observed to lose their tone and vasomotion at irreversibility although at 15 min after hemorrhage they exhibited enhanced vascular activity. Slowing of flow was seen to occur in the large venules (100 μm) and late in shock in smaller venules (25 μm). Venular dilatation was adjudged to be the vascular defect associated with the onset of irreversibility. Muscle surface pH and P O 2 followed a course similar to that seen in unanesthetized subjects. The red cell aggregation seen in the venules during the low flow state was generally reversed after reinfusion of the shed blood and restoration of arterial pressure.

The rheology of red blood cell aggregates

Abstract The aggregation and disaggregation of red blood cells is a critical variable in the rheology of blood in both physiologic and pathologic conditions. A direct visualizing viscometer (Rheoscope) was used to analyze the formation, dispersion, and flow behavior of aggregates under quantified flow conditions. Pathologic aggregates differed from normals in that they were considerably more resistant to shear and settled more rapidly. Red cells from acutely ill patients formed typical aggregates in serum indicating that serum proteins rather than fibrinogen alone are involved in the development of aggregates in pathological conditions. The elasticity and flow behavior of the aggregates were altered by osmotic swelling and crenation of the cells, which had profound effects upon aggregate formation and deformation. Their resistance to shear was not affected. These studies indicate that while red cell aggregation is a physiologic process, there are qualitative as well as quantitative differences between normal and pathologic aggregation, and that these differences are related to changes in red cell size and shape, plasma proteins, and the ambient flow (shear) forces within the circulation.

Microcirculation and coronary blood flow

Abstract The microcirculation of the coronary vasculature functions as a penetrating vascular network running from epicardium to endocardium with direct communications of both arterioles and venules with the ventricular chambers. The degree of capillary flow and utilization reflect the fact that the heart is a constantly functioning muscle. The cyclical compression of this vascular system also sets it apart from the conditions in the remainder of the systemic circulation. There are considerable species differences in the number of collateral vessels. Response to stress, such as hypoxia or arterial narrowing, involves conversion of collateral microvessels into communicating arteries, a reaction which can be seen to occur within days of the initial stimulus.

Blood Flow in the Microcirculation of the Conjunctival Vessels of Man

H. MARICQ, M.D., Fellow in the PostDoctoral Research Training Program, Department of Psychiatry, Columbia University, College of Physicians and Surgeons, New York, N. Y. and Research Associate, Essex County Overbrook Hospital, Cedar Grove, New Jersey Nailfold capillary bed, the area most frequently used for observation on human microcirculation has been reported to be remarkably constant in morphology over long periods of time. The total area of the nailfold examined by capillary microscopy and the focus of interest have varied somewhat from one study to another but have mainly been limited to the end row loops. In the present study the nailfold capillary bed was examined in 406 schizo-

Flow Behavior of Red Cells in Pathologic Sera: Existence of a Yield Shear Stress in the Absence of Fibrinogen

One of the longest controversies in the history of blood rheology concerned the question whether red cell aggregation was exclusively a pathological event as maintained by KNISELY (1) or also a physiological event. Supporting the latter assumption taken originally by FAHRAEUS (2), it has been shown that red cell aggregation is not only a physiological event both in vivo and in vitro (3,4) but a prime factor in the low shear rheology of blood (5). The role of red cell charge in red cell aggregation remains unsettled, however, it seems certain that it cannot preclude rouleaux formation in the presence of fibrinogen. The rouleaux network leads to a pronounced increase of blood viscosity near stasis as well as to the existence of yield shear stress of blood (6).

Blood flow in the microvasculature of the conjunctiva of man.

Cinephotomicrography of capillary blood flow in conjunctival vessels of man were carried out at 116 pictures per second. Red cells did not move in streamlines but rather in a heterogenous mixing type of flow. Flow in vessels of patients whose red cells were grossly aggregated was very slow and often stagnant.