Biorheology

The rate of packing of the red blood cell column during centrifugation and the haematocrit have been found to depend on the cell flexibility. This has been altered by addition of formaldehyde, heating at 49°, pH, removal of oxygen and aldosterone. Measurements of the haematocrit by dilution of haemoglobin and I¹³¹ labelled serum albumin solutions, added to these cell suspensions, did not show the same differences. An analysis of the data suggests that compression of the cells occurs during centrifugation, due to the weight of the packed cells pressing down on each other. Compared to haematocrit values for fresh whole blood of 45.5 ± 1.5 per cent obtained by centrifuging, the true haematocrit is estimated to be at least 52.5 ± 2.5 per cent.

The flow of red cell suspensions ranging in hematocrit from 0 to 45 were studied in tube diameters ranging from 72 μ to 2000 μ Suspension viscometry was determined by cone and plate viscometers between shear rates of 1 sec⁻¹ and 1500 sec⁻¹. The characteristics of an idealized marginal layer were calculated from pressure-flow rate measurements.

The distribution of cells determined by Palmer [Am. J. Physiol. 209, 1115, 1965], in a 35 μ rectangular tube, was adapted to the calculation of flow rates over a wide range of conditions. Remarkably close agreement was observed with experimental data for hematocrits below 40 per cent. For hematocrits above 40 in tube diameters greater than 155 μ, radial cell distribution does not occur to any great extent and a laminar flow equation, which includes the effect of yield stress first suggested by Scott Blair and Reiner [Nature, Lond. 184, 354, 1959], describes flow up to a Reynolds number of 800.

Poiseuille’s equation applies when the dimensionless group (yield stress)/(velocity)² density is less than 5 × 10⁻⁴ and tube diameters are greater than 155 μ and hematocrits are greater than 40.

A large scale fluid-mechanical model has been used to study certain aspects of the flow of blood in capillaries. In the model, large, rigid, neutrally buoyant disc-like cells (discoids) are transported by a viscous fluid through a 1 cm glass tube. Observations of the orientation of the model cells lead to the conclusion that both cross-sectional profile and large cell-to-tube diameter ratio are important factors in the normal (“piston-like”) stability of discoids in low Reynolds number tube flow. Pressure drop measurements show that size (i.e., diameter and thickness), spacing and orientation of discoids are factors which determine the additional pressure drop (in excess of the Poiseuille pressure drop) associated with the cell motion. For discoids in the normal orientation, the additional pressure drop is most sensitive to the diameter of the discoid. Pressure drops calculated from the experimental data are compared with pressure-drop measurements in vivo. Finally, a possible mechanism for non-linear pressure-flow characteristics for blood flow in capillaries is suggested.

A report is given on a microcirculatory phenomenon which can be provoked by application of high-concentrated oxygenation. It occurs in the retinal vascular system in very young cats. Although this phenomenon shows typical criteria of plasma-skimming in the capillary bed, it is accompanied by well-marked changes in the main vessels, i.e., a narrowing of the axial red-cell stream and the widening of the marginal plasma zone in the lumen of vessels. The basic suggestion of the genesis of this phenomenon is discussed.