Microhemodynamics of blood flow in narrow glass capillaries of 9 to 20 micrometers; the Fahraeus effect

Velocities of the red blood cell (RBC) and the suspending medium in glass capillaries of 9 to 20 micron were measured under microscopic observation. The effects of physical factors such as driving pressure, capillary diameter, hematocrits and RBC deformability on flow velocities were studied using freshly drawn blood of the rat resuspended in phosphate buffered saline solution in the hematocrit range between 5 and 12.5%. These RBC suspensions were made to flow through the test glass capillaries under known negative driving pressures. Ratios of capillary hematocrit to feed hematocrit taken as measures of the Fahraeus effect showed almost constant value of about 0.74. While, ratios of capillary hematocrit to discharge hematocrit showed a characteristic dependence on capillary diameter, showing minimal values at about 13 micron in capillary diameter. The same hematocrit ratios were found to be well correlated with values of wall shear rates estimated from the relative RBC velocities.     

Effects of erythrocyte flexibility on microvascular perfusion and oxygenation during acute anemia

Responses to exchange transfusion using red blood cells (RBCs) with normal and reduced flexibility were studied in the hamster window chamber model during acute moderate isovolemic hemodilution to determine the role of RBC membrane stiffness in microvascular perfusion and tissue oxygenation. Erythrocyte stiffness was increased by 30-min incubation in 0.02% glutaraldehyde solution, and unreacted glutaraldehyde was completely removed. Filtration pressure through 5-microm pore size filters was used to quantify stiffness of the RBCs. Anemic conditions were induced by two isovolemic hemodilution steps using 6% 70-kDa dextran to a hematocrit (Hct) of 18% (moderate hemodilution). The protocol continued with an exchange transfusion to reduce native RBCs to 75% of baseline (11% Hct) with either fresh RBCs (RBC group) or reduced-flexibility RBCs (GRBC group) suspended in 5% albumin at 18% Hct; a plasma expander (6% 70-kDa dextran; Dex70 group) was used as control. Systemic parameters, microvascular perfusion, capillary perfusion [functional capillary density (FCD)], and oxygen levels across the microvascular network were measured by noninvasive methods. RBC deformability for GRBCs was significantly decreased compared with RBCs and moderate hemodilution conditions. The GRBC group had a greater mean arterial blood pressure (MAP) than the RBC and Dex70 groups. FCD was substantially higher for RBC (0.81 +/- 0.07 of baseline) vs. GRBC (0.32 +/- 0.10 of baseline) and Dex70 (0.38 +/- 0.10 of baseline) groups. Microvascular tissue Po(2) was significantly lower for Dex70 and GRBC vs. RBC groups and the moderate hemodilution condition. Results were attributed to decreased oxygen uploading in the lungs and obstruction of tissue capillaries by rigidified RBCs, indicating that the effects impairing RBC flexibility are magnified at the microvascular level, where perfusion and oxygenation may define transfusion outcome.     

Impaired deformability of copper-deficient neutrophils

We have previously shown that dietary copper deficiency augments neutrophil accumulation in the lung microvasculature. The current study was designed to determine whether a diet deficient in copper promotes neutrophil chemoattraction within the lung vasculature or if it alters the mechanical properties of the neutrophil, thus restricting passage through the microvessels. Sprague-Dawley rats were fed purified diets that were either copper adequate (6.3 microg Cu/g diet) or copper deficient (0.3 microg Cu/g diet) for 4 weeks. To assess neutrophil chemoattraction, bronchoalveolar lavage fluid was assayed for the neutrophil chemokine macrophage inflammatory protein-2 (MIP-2) by enzyme-linked immunosorbent assay. Neutrophil deformability was determined by measuring the pressure required to pass isolated neutrophils through a 5-microm polycarbonate filter. The MIP-2 concentration was not significantly different between the dietary groups (Cu adequate, 435.4 +/- 11.9 pg/ml; Cu deficient, 425.6 +/- 14.8 pg/ml). However, compared with controls, more pressure was needed to push Cu-deficient neutrophils through the filter (Cu adequate, 0.150 +/- 0.032 mm Hg/sec; Cu deficient, 0.284 +/- 0.037 mm Hg/sec). Staining of the filamentous actin (F-actin) with FITC-Phalloidin showed greater F-actin polymerization and shape change in the Cu-deficient group. These results suggest that dietary copper deficiency reduces the deformability of neutrophils by promoting F-actin polymerization. Because most neutrophils must deform during passage from arterioles to venules in the lungs, we propose that copper-deficient neutrophils accumulate in the lung because they are less deformable.     

Hematology of the lesser mouse lemur (Microcebus murinus). A preliminary study

Some blood values of 86 lesser mouse lemurs (Microcebus murinus) maintained in captivity are presented. These are comparable with those of other prosimians: PCV 51.7%, RBC 9.2 X 10(6)/mm3, MCV 57.5 micron3, WBC 12,900/mm3, lymphocytes 59.7%, neutrophils 30.7%, monocytes 4.8%, eosinophils 4.2%, basophils 0.6%. We noted no significant difference between the sexes except that the percentage of lymphocytes was slightly lower in the females (with a corresponding percentage of neutrophils slightly higher). The lymphocyte:neutrophil ratio decreases with age, particularly in young animals; the hematocrit does not vary in adults.     

Efficiency of several micro-fiber glass filters for recovery of poliovirus from tape water.

Micro-fiber glass filters from Gelman, Filterite, Johns-Manville, and Whatman were compared with Millipore membrane filters on the basis of their virus adsorbancy, flow rate, clogging resistance, and virus concentration efficiency by using tap water at 2 nephelometric turbidity units. As virus adsorbants the Johns-Manville D39, Filterite 0.25-micron, Filterite 0.45-micron, and Millipore 0.45-micron filters were the most efficient, retaining more than 99% of the added virus in water at pH 3.5 and 0.0005 M aluminum chloride. The Johns-Manville D79 and D49 filters retained 92 and 96% of the virus, respectively, whereas the Whatman GF-D, Whatman GF-F, Gelman A-E, and Millipore AP-20 filters retained only 28, 78, 56, and 34% of the virus, respectively. The best flow rate and clogging resistance were obtained with the Johns-Manville D79 filter or with this filter acting as a prefilter to the Johns-Manville D49, Johns-Manville D39, or Filterite 0.45-micron filter. Finally, poliovirus experimentally seeded in 20 liters of tape water was recovered from Johns-Manville D79-Johns-Manville D39 or Johns-Manville D79-Filterite 0.45 micron 142-mm filter combinations was a efficiencies of 86 and 85%, respectively.     

Neutrophil maturation and activation determine anatomic site of clearance from circulation

The long-term disposition of circulating neutrophils and the site of disappearance from circulation remain unclear. We investigated neutrophil localization in mice using (111)In-labeled murine peripheral blood neutrophils, mature bone marrow neutrophils, and peritoneal exudate neutrophils to track in vivo localization of these different cell populations. Infused peripheral neutrophils were found to localize equally between liver and marrow sites by 4 h (31.2 +/- 1.9 vs. 31.9 +/- 1.8%), whereas exudate neutrophils predominantly localized to liver (42.0 +/- 1.1%) and marrow-derived neutrophils to the marrow (65.9 +/- 6.6%) where they were found to localize predominantly in the hematopoietic cords. Stimulation of marrow neutrophils before infusion caused a shift in localization from marrow to liver, and subsequent induction of an inflammatory site after infusion and marrow sequestration led to remobilization of infused marrow neutrophils but not of peripheral neutrophils. These results indicate that the marrow participates in removing neutrophils from circulation, with evidence supporting both storage and perhaps disposal functions. Furthermore, models for circulating neutrophil homeostasis should consider that the site of retention is governed by the maturation and activation states of the cell.     

Blood viscosity and optimal hematocrit in narrow tubes

Blood viscosity in normal adults was measured in glass tubes with diameters of 50, 100 and 500 microns for a wide range of adjusted feed hematocrits (15-70%). Blood viscosity decreased at each of the adjusted feed hematocrits when going from a 500-micron tube to a 50-micron tube. The viscosity reduction increased with increasing hematocrit. The steepness in the hematocrit-viscosity curves decreased with decreasing tube diameter. Erythrocyte transport efficiency (hematocrit/blood viscosity) was calculated to estimate the optimal hematocrit for oxygen transport. Optimal hematocrit averaged 38% in 500-micron tubes, 44% in 100-micron tubes and 51% in 50-micron tubes. Our results suggest that the strong F?hraeus-Lindqvist effect at high hematocrits may help to maintain oxygen transport in polycythemic patients as long as the driving pressure is sufficient.     

Computational modeling of RBC and neutrophil transit through the pulmonary capillaries.

A computational model of the pulmonary microcirculation is developed and used to examine blood flow from arteriole to venule through a realistically complex alveolar capillary bed. Distributions of flow, hematocrit, and pressure are presented, showing the existence of preferential pathways through the system and of large segment-to-segment differences in all parameters, confirming and extending previous work. Red blood cell (RBC) and neutrophil transit are also analyzed, the latter drawing from previous studies of leukocyte aspiration into micropipettes. Transit time distributions are in good agreement with in vivo experiments, in particular showing that neutrophils are dramatically slowed relative to the flow of RBCs because of the need to contract and elongate to fit through narrower capillaries. Predicted neutrophil transit times depend on how the effective capillary diameter is defined. Transient blockage by a neutrophil can increase the local pressure drop across a segment by 100--300%, leading to temporal variations in flow and pressure as seen by videomicroscopy. All of these effects are modulated by changes in transpulmonary pressure and arteriolar pressure, although RBCs, neutrophils, and rigid microspheres all behave differently.     

Effects of aging on capillary geometry and hemodynamics in rat spinotrapezius muscle

The effects of aging on muscle microvascular structure and function may play a key role in performance deficits and impairment of O2 exchange within skeletal muscle of senescent individuals. To determine the effects of aging on capillary geometry, red blood cell (RBC) hemodynamics, and hematocrit in a muscle of mixed fiber type, spinotrapezius muscles from Fischer 344 x Brown Norway hybrid rats aged 6-8 mo [young (Y); body mass 421 +/- 10 g, n = 6] and 26-28 mo [old (O); 561 +/- 12 g, n = 6] were observed by high-resolution transmission light microscopy under resting conditions. The percentage of RBC-perfused capillaries (Y: 78 +/- 3%; O: 75 +/- 2%) and degree of tortuosity and branching (Y: 13 +/- 2%; O: 13 +/- 2%, additional capillary length) were not different in O vs. Y muscles. Lineal density of RBC-perfused capillaries in O was significantly reduced (Y: 30.7 +/- 1.8, O: 22.8 +/- 3.1 capillaries/mm; P < 0.05). However, RBC-perfused capillaries from O rats (n = 78) exhibited increased RBC velocity (VRBC) (Y: 219 +/- 12, O: 310 +/- 14 microm/s; P < 0.05) and RBC flux (FRBC) (Y: 27 +/- 2, O: 41 +/- 2 RBC/s; P < 0.05) vs. Y rats (n = 66). Thus O2 delivery per unit of muscle was not different between groups (Y: 894 +/- 111, O: 887 +/- 118 RBC. s-1. mm muscle-1). Capillary hematocrit was not different in Y vs. O rats (Y: 26 +/- 1%, O: 28 +/- 1%: P > 0.05). These data indicate that in resting spinotrapezius muscle, aging decreases the lineal density of RBC-perfused capillaries while increasing mean VRBC and FRBC within those capillaries. Whereas muscle conductive O2 delivery and capillary hematocrit were unchanged, elevated VRBC reduces capillary RBC transit time and may impair the diffusive transport of O2 from blood to myocyte particularly under exercise conditions.     

The microcirculation of myocutaneous island flaps in pigs studied with radioactive blood volume tracers and microspheres of different sizes.

In order to further improve the understanding of hemodynamic changes in the immediate postoperative phase after elevation of myocutaneous flaps, regional blood flow and arteriovenous (A-V) shunting were measured in rectus abdominis island flaps in 8 pigs. Radioactive microspheres of two sizes (15 and 50 micron) were used. Approximately half (53.4 +/- 6 percent) of the 15-micron microspheres and one-fourth (24.1 +/- 6 percent) of the 50-micron microspheres entering the flap appeared in the venous outflow. Compared with the control area, A-V shunting was significantly increased in muscle and substantially more pronounced in skin. Nutritional blood flow, total blood flow, and vascular volume were increased in muscle and unchanged in skin and subcutis. The lowest tissue hematocrit of 7 +/- 1 percent was found in skin as compared with a central hematocrit of 35 +/- 2 percent. Tissue hematocrit in flap muscle was decreased to 17 +/- 2 percent when compared with control muscle (22 +/- 3 percent), and the mean transit time for blood was correspondingly decreased. Thus vasodilation provided increased perfusion through muscular capillaries and through A-V shunts. Shunting of 15-micron microspheres appeared to take place not only in skin, but also in subcutis and muscle, which challenges the widespread belief that A-V shunting does not occur in muscle.     

Wound fiberglass depth filters as a less expensive approach for the concentration of viruses from water

Wound fiberglass depth cartridge filters (25.4 cm) with a nominal porosity of 1 micron were used to concentrate viruses from large volumes of surface water. They were found to be an excellent, less expensive alternative to the 0.2-micron pleated cartridge filters normally used for the concentration of enteric viruses from water. More than 99% of experimentally seeded poliovirus was adsorbed to these filters when the pH of the water was adjusted to pH 3.5 and aluminium chloride was added to a final concentration of 0.001 M, as recommended for electronegative filters. In comparative recovery of indigenous viruses from river water, similar results were obtained with two 1-micron or a 3-microns + 0.2-micron filter combination. The cost of the two 1-micron filters is about Can. $26, while it is about Can. $58 for the other combination.     

A new red blood cell filtration device with improved time resolution and its application to the impaired RBC deformability in the diabetic ob/ob mouse

A new filtration device and blood handling technique for the assessment of RBC deformability in small blood samples is described and used to study RBC deformability in adult obese-hyperglycemic ob/ob-mice and normoglycemic controls. The new filtration device was designed to improve the time resolution during RBC incubation. Test and control RBC suspensions were directly filtered from two identical incubation chambers under a constant pressure of 1200 Pa. Nuclepore filters (3 microns) were mounted on top of several standard test tubes into which the filtrate was subsequently collected and weighed. Because the RBCs were resuspended to a very low (0.01%) hematocrit, the average number of RBCs passing each pore was less than 10. Therefore, any detectable difference must reflect the physical properties of RBCs, e.g. shape or viscoelasticity, whereas the role of white blood cells is negligible. When ob/ob-mouse RBCs were studied with the new technique they showed impaired filtrability as compared with control RBCs, both when incubated without glucose and with glucose present at the same concentration as that recorded in the RBC donating mouse.     

In vivo neutrophil sequestration within lungs of humans is determined by in vitro "filterability".

Neutrophils are normally delayed in transit through the lung microcirculation, relative to the passage of erythrocytes. This sequestration contributes to a pulmonary pool of neutrophils that may relate to the relative inability of neutrophils to deform compared with erythrocytes when in transit in the pulmonary capillaries. A micropore membrane was used to model the human pulmonary microcirculation, in which cell deformability was measured as the pressure developed during filtration of the cells through the membrane at a constant flow. We demonstrated a significant correlation between in vitro deformability and in vivo lung sequestration of indium-111-labeled neutrophils in 10 normal subjects (r = 0.69, P less than 0.02). In eight patients with stable chronic obstructive pulmonary disease, this relationship was not significant (r = -0.2, P greater than 0.05). Furthermore, in a subject with microscopic pulmonary telangiectasia known to allow significant passage of 30-microns microspheres, neutrophils passed through the lungs without delay. Moreover, neutrophils from patients studied acutely with an exacerbation of chronic obstructive pulmonary disease were temporarily less deformable (P less than 0.01). These studies confirm that cell deformability is an important determinant of the normal neutrophil sequestration within the lungs. Changes in cell deformability may alter the extent of this sequestration.     

Pulmonary neutrophil kinetics in sheep: effects of altered hemodynamics

We investigated the effect of elevated left atrial pressure and reduced cardiac output on pulmonary neutrophil kinetics in the sheep. Sheep neutrophils were isolated, labeled with 111In-oxine, and reinfused. Erythrocytes were labeled with [99mTc]pertechnetate. A gamma camera measured the lung activities of the labeled neutrophils and erythrocytes. The results indicated that 38.5% of the total injected neutrophils marginated in the lung. Pulmonary hemodynamics were altered by inflating a left atrial balloon three times in each sheep for 15-30 min to achieve 5- to 25-mmHg increments in pulmonary arterial wedge pressure. At least a 30-min recovery period was allowed between inflations. After each left atrial balloon inflation, neutrophil uptake remained unchanged from base line, despite decreased mean cardiac output to 0.67 +/- 0.24 (+/- SD) 1/min and increased pulmonary blood volume. The absence of pulmonary neutrophil uptake was confirmed by arterial-venous measurements. Increased pulmonary blood volume had little effect on lung neutrophil uptake, suggesting that most of the pulmonary neutrophils are marginated. We conclude that the lungs have a large marginated neutrophil pool compared with the circulating pool and that reduced cardiac output and elevated left atrial pressure have no effect on pulmonary neutrophil kinetics in the sheep.     

Passive mechanical behavior of human neutrophils: power-law fluid.

The mechanical behavior of the neutrophil plays an important role in both the microcirculation and the immune system. Several laboratories in the past have developed mechanical models to describe different aspects of neutrophil deformability. In this study, the passive mechanical properties of normal human neutrophils have been further characterized. The cellular mechanical properties were assessed by single cell micropipette aspiration at fixed aspiration pressures. A numerical simulation was developed to interpret the experiments in terms of cell mechanical properties based on the Newtonian liquid drop model (Yeung and Evans, Biophys. J., 56: 139-149, 1989). The cytoplasmic viscosity was determined as a function of the ratio of the initial cell size to the pipette radius, the cortical tension, aspiration pressure, and the whole cell aspiration time. The cortical tension of passive neutrophils was measured to be about 2.7 x 10(-5) N/m. The apparent viscosity of neutrophil cytoplasm was found to depend on aspiration pressure, and ranged from approximately 500 Pa.s at an aspiration pressure of 98 Pa (1.0 cm H2O) to approximately 50 Pa.s at 882 Pa (9.0 cm H2O) when tested with a 4.0-micron pipette. These data provide the first documentation that the neutrophil cytoplasm exhibits non-Newtonian behavior. To further characterize the non-Newtonian behavior of human neutrophils, a mean shear rate gamma m was estimated based on the numerical simulation. The apparent cytoplasmic viscosity appears to decrease as the mean shear rate increases. The dependence of cytoplasmic viscosity on the mean shear rate can be approximated as a power-law relationship described by mu = mu c(gamma m/gamma c)-b, where mu is the cytoplasmic viscosity, gamma m is the mean shear rate, mu c is the characteristic viscosity at characteristic shear rate gamma c, and b is a material coefficient. When gamma c was set to 1 s-1, the material coefficients for passive neutrophils were determined to be mu c = 130 +/- 23 Pa.s and b = 0.52 +/- 0.09 for normal neutrophils. The power-law approximation has a remarkable ability to reconcile discrepancies among published values of the cytoplasmic viscosity measured using different techniques, even though these values differ by nearly two orders of magnitude. Thus, the power-law fluid model is a promising candidate for describing the passive mechanical behavior of human neutrophils in large deformation. It can also account for some discrepancies between cellular behavior in single-cell micromechanical experiments and predictions based on the assumption that the cytoplasm is a simple Newtonian fluid.     

Signaling functions of L-selectin in neutrophils: alterations in the cytoskeleton and colocalization with CD18.

Ligation and clustering of L-selectin by Ab ("cross-linking") or physiologic ligands results in activation of diverse responses that favor enhanced microvascular sequestration and emigration of neutrophils. The earliest responses include a rise in intracellular calcium, enhanced tyrosine phosphorylation, and activation of extracellular signal-regulated kinases. Additionally, cross-linking of L-selectin induces sustained shape change and activation of beta2 integrins, leading to neutrophil arrest under conditions of shear flow. In this report, we examined several possible mechanisms whereby transmembrane signals from L-selectin might contribute to an increase in the microvascular retention of neutrophils and enhanced efficiency of emigration. In human peripheral blood neutrophils, cross-linking of L-selectin induced alterations in cellular biophysical properties, including a decrease in cell deformability associated with F-actin assembly and redistribution, as well as enhanced adhesion of microspheres bound to beta2 integrins. L-selectin and the beta2 integrin became spatially colocalized as determined by confocal immunofluorescence microscopy and fluorescence resonance energy transfer. We conclude that intracellular signals from L-selectin may enhance the microvascular sequestration of neutrophils at sites of inflammation through a combination of cytoskeletal alterations leading to cell stiffening and an increase in adhesiveness mediated through alterations in beta2 integrins.     

Alteration of blood hemorheologic properties during cerebral ischemia and reperfusion in rats

The cerebral ischemia and reperfusion rat model was employed in this experiment to study the rheological properties (i.e. viscosity, hematocrit, red blood cell deformability and thixotropic properties) of whole blood. The results of this study show that a significant relation exists between the duration of cerebral ischemia and reperfusion and the viscosity, hematocrit and thixotropic parameters of whole blood, but there is no significant influence on the deformability of RBC. Blood viscosity values declined gradually throughout the ischemia period, e.g., after 1h of ischemia, the values of whole blood viscosity under high, middle and low shear rates were 44, 28 and 23% lower than normal, respectively. Whereas after 1h of reperfusion, the values of viscosity increased rapidly to values 160, 57 and 41% higher than normal under the high, middle and low levels of shear rate, while the viscosity values after 12h of reperfusion tended to return to normal values. The values of hematocrit H and thixotropic parameter tau(0) and mu also gradually declined with the increase in the duration of ischemia, but increased significantly after 1h of reperfusion. The values of H, tau(0) and mu after 1h of reperfusion are significantly greater than that in the period of cerebral ischemia, the value of H, tau(0) is also higher than normal. With the increase in reperfusion time, H, tau(0) gradually returned to normal level, at the same time, mu also decreased.     

Stiffened erythrocytes augment the pulmonary hemodynamic response to hypoxia

Isolated rat lungs were perfused with suspensions containing normal and stiffened erythrocytes (RBCs) during normoxic and hypoxic ventilation to assess the effect of reduced RBC deformability on the hypoxic pressor response. RBC suspensions were prepared with cells previously incubated in isotonic phosphate-buffered saline with or without 0.0125% glutaraldehyde. The washed RBCs were resuspended in isotonic bicarbonate-buffered saline (with 4% albumin) to hematocrits of approximately 35%. The lungs were perfused with control and experimental cell suspensions in succession while pulmonary arterial pressure was measured during normoxic (21% O2) and hypoxic (3% O2) ventilation. On the attainment of a peak hypoxic pressor response, flow rate was changed so that pressure-flow curves could be constructed for each suspension. RBC deformability was quantified by a filtration technique using 4.7-microns-pore filters. Glutaraldehyde treatment produced a 10% decrease in RBC deformability (P less than 0.05). Over the range of flow rates, Ppa was increased by 15-17% (P less than 0.05) and 26-31% (P less than 0.05) during normoxic and hypoxic ventilation, respectively, when stiffened cells were suspended in the perfusate. The magnitude of the hypoxic pressor response was 50-54% greater with stiffened cells over the three flow rates. In a separate set of experiments, normoxic and hypoxic arterial blood samples from conscious unrestrained rats were used to investigate the effects of acute hypoxia on RBC deformability. Deformability was measured with the same filtration technique. There was no difference in the deformability of hypoxic compared with normoxic RBCs. We conclude that the presence of stiffened RBCs enhances the hemodynamic response to hypoxia but acute hypoxia does not affect RBC deformability.     

Shedding of the endothelial glycocalyx in arterioles, capillaries, and venules and its effect on capillary hemodynamics during inflammation

The endothelial glycocalyx has been identified as a barrier to transvascular exchange of fluid, macromolecules, and leukocyte-endothelium [endothelial cell (EC)] adhesion during the inflammatory process. Shedding of glycans and structural changes of the glycocalyx have been shown to occur in response to several agonists. To elucidate the effects of glycan shedding on microvascular hemodynamics and capillary resistance to flow, glycan shedding in microvessels in mesentery (rat) was induced by superfusion with 10(-7) M fMLP. Shedding was quantified by reductions of fluorescently labeled lectin (BS-1) bound to the EC and reductions in thickness of the barrier to infiltration of 70-kDa dextran on the EC surface. Red cell velocities (two-slit technique), pressure drops (dual servo-null method), and capillary hematocrit (direct cell counting) were measured in parallel experiments. The results indicate that fMLP caused shedding of glycans in all microvessels with reductions in thickness of the barrier to 70-kDa dextran of 110, 80, and 123 nm, in arterioles, capillaries, and venules, respectively. Intravascular volumetric flows fell proportionately in all three divisions in response to rapid obstruction of venules by white blood cell (WBC)-EC adhesion, and capillary resistance to flow rose 18% due to diminished deformability of activated WBCs. Capillary resistance fell significantly 26% over a 30-min period, as glycans were shed from the EC surface to increase effective capillary diameter, whereas capillary hematocrit and anatomic diameter remained invariant. This decrease in capillary resistance mitigates the increase in resistance due to diminished WBC deformability, and hence these concurrent rheological events may be of equal importance in affecting capillary flow during the inflammatory process.     

The pressure dependent dynamic elasticity of 35 thoracic and 16 abdominal human aortas in vitro described by a five component model

Segments of 35 thoracic and 16 abdominal human aortas, including nine pairs, aged 30-78 yr at autopsy, were perfused with 37 degrees C Tyrode's solution at in situ length. Diameter changes due to 20 mmHg pressure steps between 20 and 180 mmHg were measured to 1 micron accuracy at an equivalent noise level of 0.1 micron RMS, using balanced transducers. Aortic creep curves at each pressure level were described individually by a constant plus bi-exponential creep model characterized by two creep fractions (alpha 1 and alpha 2) and two time constants (tau 1 and tau 2). Creep fractions and time constants increased substantially with the pressure level, indicating a significant effect of pressure or distension on aortic viscoelasticity. At 110 mmHg the mean +/- 1 S.D. parameter values were: thoracic aorta: alpha 1 = 0.076 +/- 0.017, alpha 2 = 0.102 +/- 0.028, tau 1 = 0.73 +/- 0.29 s, tau 2 = 14.0 +/- 4.1 s; abdominal aorta: alpha 1 = 0.078 +/- 0.017, alpha 2 = 0.101 +/- 0.025, tau 1 = 0.61 +/- 0.12 s, tau 2 = 12.1 +/- 3.4 s. Nine paired comparisons at each pressure level showed that creep fractions and time constants of thoracic and abdominal segments were not significantly different (p = 0.05).