Plasma and platelet skimming at T-junctions.

Separation of plasma, erythrocytes, and platelets was studied in T-shaped bifurcations prepared by a casting technique. The diameters of the side and main channel were 100 microns and 200 microns, respectively. Small blood samples (about 20 microliters) were collected from the side channel for determination of erythrocyte and platelet concentrations which were compared to the corresponding values in the reservoir. Conversely to the well-known erythrocyte reduction due to plasma skimming a relative enrichment of platelets in the side branch was observed. The thickness of the plasma layer in the main channel was found to depend on the aggregation tendency of the erythrocytes for Reynolds numbers around 0.3.

Fåhraeus-Vejlens effect: margination of platelets and leukocytes in blood flow through branches.

The distribution of platelets and leukocytes was studied in tube flow at T-junctions. The diameters were 200 microns for the main channel and 100 microns for the side branch, or 100 microns and 80 microns, respectively. The concentrations of platelets and leukocytes in the blood collected from the side branch and in the blood from the reservoir were measured using blood samples with different levels of fibrinogen. The concentration of platelets in the blood from the side branch was found to be about 1.2-1.5 times higher than in the reservoir. This ratio was found dependent on the feed hematocrit and fibrinogen concentration. The concentration of leukocytes in the blood from the side branch was about 0.9-1.4 times the value of the blood in the reservoir depending slightly on the fibrinogen concentration.

Theoretical model of phase separation of erythrocytes, platelets, and plasma at branches.

Analytical expressions for the changes of the erythrocyte and platelet concentration are calculated as blood flows through a single bifurcation. The effect of the separation surface, which divides the blood into the two daughter vessels, upon the concentration changes is studied. Numerical fitting to the experimental data shows that both the erythrocyte and platelet distributions across the vessel are nonuniform. The maximum of the erythrocyte concentration is in the core, and of the platelet concentration, in the vicinity of the wall. A separation surface that is convex towards the axis of the vessel yields better fit to the experimental data than a concave one. Together with other studies, this suggests that flow-divider geometry depends on flow conditions and the vessel diameters. The results can be used for estimation of the distribution of erythrocytes and platelets in the microvasculature.

Theoretical analysis of regional blood flow studies.

Regional blood flow measurements were analyzed using a two-compartment model. The distribution of injected tracer between blood and tissue was assumed to be in equilibrium only at the beginning of the experiment. Equations for the determination of the partition coefficient of injected radionuclides from in vivo measurements were derived. These equations can also be used for estimation of the relative blood content of a tumor or an organ. The results were applied to experiments, where three diffusible radionuclides were injected simultaneously into patients with diagnosed tumors.

Regional blood flow in human tumours measured with argon, krypton and xenon.

Blood perfusion in 19 superficial tumour metastases was measured by injecting three tracers, 41Ar, 85Krm and 135Xe or 133Xe, simultaneously into the tissue. The blood flow rate was calculated from the constants of the biexponential functions fitted to the measured counts. The mean blood flow in the 19 tumours studied was 18 ml/(100 g min) (SD +/- 10). Alternatively, the relative partition coefficients of argon, krypton and xenon were calculated from the equations obtained using compartmental analysis. The mean values for the ratios of the partition coefficients of these nuclides were found to be between 0.82 and 1.07.

Partition coefficient ratios and tumour perfusion studied with 85mKr and 133Xe.

The relative partition coefficients of krypton and xenon, and the regional blood flow in 27 superficial malignant tumour nodules in 22 patients with diagnosed tumours were measured using the 85mKr- and 133Xe-clearance method. In order to minimize the effect of biological variables on the measurements the radionuclides were injected simultaneously into the tumour. The distribution of the radiotracers was assumed to be in equilibrium at the beginning of the experiment. The blood perfusion was calculated by fitting a two-exponential function to the measuring points. The mean value of the perfusion rate calculated from the xenon results was 13 +/- 10 ml/(100 g.min) [range 3 to 38 ml/(100 g.min)] and from the krypton results 19 +/- 11 ml/(100 g.min) [range 5 to 45 ml/(100 g.min)]. These values were obtained, if the partition coefficients are equal to one. The equations obtained by using compartmental analysis were used for the calculation of the relative partition coefficient of krypton and xenon. The partition coefficient of krypton was found to be slightly smaller than that of xenon, which may be due to its smaller molecular weight.

The relationship between growth and allometry.

The asymptotic allometric equations are derived by considering the relative growth of different physiological and anatomical quantities. The growth rate is assumed to be proportional to the value of the quantity, where the proportion coefficient depends on time. The allometric exponents are calculated for some organs of man and compared with the experimental values for primates. The exponents are generally time-dependent during the growth. The effect of the choice of the origin of time axis on the asymptotic allometric exponents is studied.

Compaction stasis due to gravitational red cell migration and floatational plasma skimming. Reversal of the fahraeus effect due to pathological RCA-formation in plastic tubes and mesenteric venules.

A horizontally aimed microscope was directed at isolated rat mesentery preparations as well as artificial microchannels cast in polyester blocks. They were perfused with aggregating red cell suspensions containing about 7 g/l of bovine fibrinogen at various perfusion pressures. The effects induced by gravitational influences were monitored by measuring the local red cell concentration (video-densitometry) and velocity profiles (IPM-dual slit velocimetry). At low perfusion pressures, sedimentation during maintained flow occurs, leading to a relative red cell slowing compared with plasma. Consequently, a progressive deposition of red cells at the bottom of vessels occurs and finally, blocking of the vessel by aggregated red cells is seen. Thus, the well-known phenomenon of compaction stasis must be attributed not merely to a transmural plasma loss but also to gravitationally induced haemoconcentration.