On-line volume flow rate and velocity profile measurement for blood in microvessels

Inaccuracies in blood flow estimates in microvessels during arteriolar vasoconstriction

The effect of vasomotor tone on blood flow estimates was evaluated in the hamster cheek pouch and cremaster muscle microcirculation. The products of arteriolar cross-sectional area and red blood cell velocity were calculated in three different cases: (1) at arteriolar bifurcations, (2) in short segments of an arteriole constricted by iontophoretic application of norepinephrine, and (3) at randomly selected second- and third-order arterioles. Vasodilation of the microcirculation was induced by topical application of adenosine. Vasoconstriction was induced by elevation of superfusion solution PO2. If true volume flow is accurately estimated by this method then: the sum of measured branch flows at a bifurcation should equal feed flow; measured flow through constricted arteriolar segments should equal flow proximal or distal to the constricted segment; and, following experimental manipulations, relative changes in estimated flow in second- and third-order arterioles should be equal. Our findings suggest that the blood flow estimates were not always accurate. The sum of branch flows was equal to feed flow only across bifurcations with low or resting vascular smooth muscle tone. During vasoconstriction, feed flow averaged 40% higher than the sum of downstream flows. In addition, estimated flow was 15% lower in constricted segments of an arteriole compared to dilated contiguous segments of the vessel. During alterations in vasomotor state, estimated fractional changes in flow in second- and third-order arterioles differed by more than sixfold. Therefore, blood flow estimates with the dual-slit method may not be reliable under conditions of high vasomotor tone. We speculate that the error may result largely from uncertainties in the diameter measurement.

Arteriolar proliferation in the rat cremaster muscle as a long-term autoregulatory response to reduced perfusion

Long-term autoregulation of arterial blood flow to sustained increases or decreases in arterial pressure has been previously reported. It is not clear whether the adaptive process at the microscopic level is a modification of vessel caliber or density. The present study was undertaken to assess morphological and hemodynamic changes resulting from a sustained decrease in perfusion to the cremaster muscle in normal rats. The main feeding arteriole to the cremaster muscle was ligated at age 3 weeks in 11 male Wistar rats. At 6 weeks, the cremaster muscles on the ligated and control sides were evaluated microscopically for hemodynamic status and vascular architecture differences. Pressure was reduced 30% on the ligated side without a decrease in volume flow. This was possible because the number of second-order arterioles increased by 48% and the number of third-order arterioles increased by 98% on the ligated side. Although twice as many third-order arterioles were present in the ligated muscle, the same percentage was closed to flow in the resting state as in the control muscle (31 vs 29%). These findings are consistent with a long-term autoregulatory process which modulates arteriolar density in response to altered perfusion or distending pressure. The same process may be responsible for arteriolar rarefaction in response to sustained elevation in arterial pressure in the cremaster muscle of the spontaneously hypertensive rat (SHR).

Arteriovenous distribution of hemodynamic parameters in the rat dental pulp

A systematic investigation of the distribution of red cell velocity throughout the hierachy of the rat pulp microvascular network was performed. Luminal diameters (D) of microvessels ranging in size from 8 to 72 micron were measured in situ by an electronic video image shearing technique. Intravascular red cell velocities (Vrbc) were simultaneously measured by a variation of the "two-slit" photometric technique and intravascular volumetric flow rates (Q) were calculated. It was found that red cell velocity decreased monotonically throughout successive arteriolar divisions to attain capillary values of 1/10 those in the feeding arterioles. A slight rise in Vrbc was found in the venous confluences, however, a maximum value of Vrbc of only 1/5 large arteriolar values was evident in the large collecting venules. In contrast, the calculated volumetric flow rate distribution was found to be nearly parabolic from arterioles (40 micron) and their paired (72 micron) collecting venules. This behavior was attributed to the dominance of microvessel cross-sectional areas as a determinant of Q.

Effect of pure O2-breathing on retinal blood flow in normals and in patients with background diabetic retinopathy

The noninvasive Laser Doppler velocimetry technique was applied in normal volunteers and in patients with background diabetic retinopathy to determine retinal blood flow during pure oxygen breathing at atmospheric pressure. In normal subjects, five minutes of oxygen breathing produced a 63 +/- 6% decrease in retinal blood flow. In the diabetic patients blood flow decreased by only 36 +/- 15%. A significant correlation between the decrease in blood flow and that of the diameter of the veins was found in the diabetic patients. This was not the case for the normal subjects.

An optical doppler intravital velocimeter

A system has been implemented for measurement of red blood cell velocity in microvessels by using an optical Doppler technique. Ronchi rulings are used to stimulate a differential grating to translate red blood cell movement to light intensity variations. These variations are sensed by two photodiodes coupled in a resistive subtraction mode. The nonelectronic subtraction allows high transimpedance gains (2 X 10(9) V/I) while noise is held to a minimum (4.5 mv RMS in a 5-kHz bandwidth). To derive average velocity the average frequency determination of the amplified signal is performed with a thresholding frequency-to-voltage functional block. The velocimeter provides the typical performance features of an optical Doppler system, including high-frequency response, without the need for the complications of the laser Doppler technique or the requirement of custom micro-prism gratings. The device represents a cost-effective approach to intravital work, and offers significant improvements in performance over standard techniques.

The application of an improved dual-slit photometric analyzer for volumetric flow rate measurements in microvessels

The dual-window photometric analyzer is currently a widely used instrument to estimate volumetric flow rates in microvessels. This instrument provides an analog voltage that is proportional to cell velocity, but the question arises how this voltage is related to the mean velocity in the blood vessel. It requires a separate investigation and in this report a calibration of an improved version of a dual-window photometric analyzer for volumetric flow rate measurements in microvessels as large as 470 microns in diameter is presented. The relationship between correlator velocity and blood mean velocity is presented in a closed empirical form. Within a 5% error of the mean it is found to be dependent on tube diameter but independent of hematocrit (between 3 and 40%). A rotating transparent disc plated with dried red blood cells was used to test the dynamic response of the instrument for frequencies up to 10 Hz. The amplitude reduction was less than 10% up to 3 Hz. Thereafter it fell off at 6 db/octave.

Insulin delays platelet accumulation in injured cerebral microvessels of diabetic and normal mice

Pial arterioles of mice were injured by exposure to a noxious light/dye stimulus and resultant accumulation of platelets at the damaged site was monitored in vivo. When normal mice were injected with insulin 2 hr before damaging the arterioles, the accumulation of platelets was significantly delayed, but single injections of even higher doses failed to alter accumulation in mice with streptozotocin diabetes. When diabetic mice were placed on a schedule of daily injections of ultra lente insulin until normoglycemic, the treatment significantly delayed the accumulation of aggregating platelets in damaged vessels.

Intestinal microvascular adaptation during maturation of spontaneously hypertensive rats

Adaptive microvascular changes in increased arterial pressure were investigated in the intestine of spontaneously hypertensive rats (SHR). In 4- to 5-week-old normal Wistar-Kyoto (WKY) and SHR ras, as well as in 18- to 21-week-old WKY rats, the number of arterioles of a given type per milligram of tissue were very similar. However, 18- to 21-week-old SHR had 30% to 35% fewer arterioles in the diameter range of 25--35 mu, as if intestinal vessels were lost or failed to grow during maturation. The largest and smallest arterioles in the intestine of adult SHR were constricted by 20% to 25%, but other vessels in the SHR had an equal or increased diameter relative to those in WKY rats. As a result of rarefaction and selective vasoconstriction in SHR, microvascular pressures in the intestinal muscle of SHR were near those in WKY rats, and those in villi of SHR were equal to those in WKY rats despite a 60% to 70% increase in mean arterial pressure in SHR. The percentage of small arterioles (less than 15 mu) that were intermittently closed to flow at rest was minimal, and the total number of small vessels per milligram of tissue was equal in WKY and SH rats. These data indicate that the adaptive changes in the intestinal vasculature of SHR do not include the loss of small arterioles as occurs in skeletal muscle but that the vascular branching pattern is disturbed, and the largest and smallest arterioles are constricted in the intestine of SHR.

Analysis of network flow distribution: computational aid to minimize experimental expenditure

Several problems in microvascular research, e.g., total blood flow and red cell flux distribution, can only be solved by evaluating the flow distribution to all branches of microvessel networks. We present a method to obtain values of volumetric blood flow and red cell flux in individual networks by taking measurements in an adequately chosen number of the vessels (50-33%) in the network. Actual measurements are not mandatory in all vessels because in a major part of them computational quantification of flows and fluxes is possible. The proposed method consists of a procedure to document in matrix form the connections of a network found in a preparation. By appropriate mathematical manipulations of the matrix, a preselection of measuring sites within the network can be achieved. From the flow values in these sites the remaining vessel flows can be calculated by utilizing the law of mass conservation in all branching points and confluxes. The worst case error of the procedure can be calculated from the errors of the single measurements. The method can also be applied, whenever, for experimental reasons (e.g., poor microscopic focus, flow too rapid or too slow), certain branches do not lend themselves for measurement. A BASIC program can be made available upon request, which carries out all calculations necessary for an appropriate selection in such a short time (about 1 min) so that it can easily be performed before the start of data acquisition.

Autoregulatory patterns in the arteriolar network of cat mesentery

The purpose of this study was to examine the distribution and extent of autoregulatory behavior among arterioles in the arcade network of the cat mesentery. We found that 71% of 169 arterioles studied dilated and 63% showed blood flow autoregulation with arterial pressure reduction to 55-60 mm Hg. There appeared to be a direct relation between the degree of dilation and the degree of autoregulation when individual vessels were compared (r = 0.71). The dilation of individual arterioles was also correlated with the degree of autoregulation of other vessels in the same field (r = 0.51). To a lesser extent, the degree of dilation of individual arterioles was correlated with the degree of dilation of other vessels in the same field (r = 0.30). In comparing vessels which dilated, the largest average response was found in preparations where all vessels dilated. On a percentage basis the small arterioles located distally in the network dilated more than the large arterioles located proximally. In preparations having both reactive and nonreactive arterioles to pressure reduction, the nonreactive vessels were usually located geographically in proximity to each other. The flow distribution in the arteriolar network remained relatively stable on pressure reduction; the total tissue area perfused by a first-order arteriole shifting by 3% on the average.

Measuring the microcirculation in the human conjunctiva bulbi under normal and hyperperfusion conditions

We present a system consisting of a slit lamp stereomicroscope and an adapted video system for the examination of the conjunctival microcirculation. This system permits measurement of the flow of erythrocytes in the vessels of the bulbar conjunctiva. We present values obtained from clinically normal individuals under normal conditions and under conditions of hyperperfusion.

Dimethyl sulfoxide effects on platelet aggregation and vascular reactivity in pial microcirculation

DMSO is a hydroxyl radical scavenger that inhibits platelet aggregation in vivo in injured microvessels, and that also inhibits the dilation displayed by pial arterioles following a local injury. The injurious stimulus is a result of local excitation of circulating sodium fluorescein by an appropriate light source. It is likely that this excitation results in the generation of hydroxyl radicals, which are the immediately injurious agent. This postulate is supported not only by the inhibitory effect of DMSO but also by the inhibitory effect of glycerol, another hydroxyl scavenger. Both the hypothesis that DMSO inhibits hydroxyl-mediated dilation, and the hypothesis that free radicals can dilate pial arterioles, are further supported by direct evidence from studies employing local application of xanthine oxidase plus acetaldehyde. This well established radical-generating system dilated pial arterioles. The dilation was inhibited by the local application of superoxide dismutase and also by local application of catalase, as well as by intraperitoneal administration of DMSO. Since DMSO failed to inhibit the dilation produced by increases of inspired CO2, we believe that the inhibitory effect of DMSO on the other dilating stimuli in these studies was due to the hydroxyl scavenging properties of this drug, rather than to other nonspecific effects.

Hydronephrosis: a new method to visualize vas afferens, efferens, and glomerular network

We have developed a new preparation for in vivo visualization of the glomerular microcirculation, the vas afferens and the vas efferens. This preparation utilizes postischemic hydronephrosis (PIH) to destroy the renal tubular system while preserving a portion of the cortex. In this preparation, glomeruli and associated vasculature remained intact. Observations can be made with either incident light or transillumination. The inner diameter of the vas afferens, measured within 50 microns of the glomerular vascular pole, was 7.9 +/- 0.5 microns (N = 12; SEM) while that of the vas efferens was 7.7 +/- 0.5 microns (N = 12). Both vessels were narrower adjacent to the glomerulus; minimal diameters in this region were 4.5 +/- 0.5 microns (N = 10) and 4.3 +/- 0.5 microns (N = 11), respectively. A specialized round cell, which may act as a sphincter, was seen in the vas efferens. In a second series of experiments, blood velocity was measured in the vas afferens and efferens about 100 microns from the vascular pole. Mean control velocities at these sites were 5.9 +/- 0.9 (N = 14) and 4.6 +/- 1.3 (N = 9) mm X sec-1, respectively; diameters at these same sites were 10.3 +/- 0.6 microns and 11.2 +/- 0.7. During angiotensin II infusion (first series, 0.2 to 0.4 micrograms X min-1 X kg-1, i.v.) the vas efferens in the vicinity of the glomerulus constricted by 22% whereas the corresponding vas afferens showed no consistent response. During angiotensin II infusion, the filtration fraction (GFR/RPF) may, therefore, be elevated by an increased resistance in the vas efferens, particularly at the outflow point of the glomerulus. In the second series of experiments higher dosages of angiotensin II caused vasoconstriction of both vessels, especially at sites more distant from the glomerulus. Furthermore, the new approach is suitable for observing the flow direction within single capillaries of one third to one half of the glomerulus. Therefore, for the first time it is possible to determine the real flow direction in a three-dimensional way.

Effect of OKY-1581, a thromboxane synthesis inhibitor, on the platelet aggregation and vasodilation induced by injury of mouse pial arterioles

OKY-1581 (sodium [E]-3-[4-(3-pyridylmethyl)-phenyl]-2-methylacrylate) is a known inhibitor of thromboxane synthesis, a class of agents thought potentially useful in treating conditions characterized by platelet aggregation. Doses of 10, 100, and 300 mg/kg were administered intraperitoneally to mice 1 h before their pial vessels were injured by a combination of light from a filtered mercury lamp and intravascular sodium fluorescein. In this model platelet aggregation and arterial dilation are produced. Pretreatment with OKY-1581 had no effect except at the 300-mg/kg dose, which enhanced aggregation as manifested by a decrease in the time required for the noxious stimulus to initiate aggregation. In addition the dilation increased in magnitude as compared with that in controls. The latter result is compatible with a decreased synthesis of thromboxane, a known constrictor of cerebral vessels. However, the enhancement of aggregation by a high dose of OKY-1581 was unexpected and paradoxical. The data do not support the use of thromboxane synthesis inhibitors as therapy for conditions caused by platelet aggregation, but the results may be dependent upon the species, the vascular bed, or the method used to induce aggregation.

In vitro and in vivo measurement of red cell velocity with epi- and transillumination

Measurement of red cell velocity with the dual-slit cross-correlation method in glass capillary tubes during transillumination indicates that the measured velocity must be divided by a correction factor of approximately 1.6 to equal the average velocity calculated from a known flow and inner diameter. Whether the same correction factor exists when red cell velocity is measured during epiillumination is questionable. Red cell velocity was measured with the dual-slit correlation method nearly simultaneously using epi- (EL) and transillumination (TL) while glass tubes (40-100 microns, i.d.) were pump perfused with whole human blood (hematocrit 39-42%). With TL, the measured velocity is 1.58 +/- 0.07 (SEM) times the calculated average velocity, whereas a factor of 2.04 +/- 0.04 (SEM) was obtained with epiillumination. When intestinal arterioles with approximately the same inner diameters and flow velocities as the glass tubes were used, the ratio of velocities measured with TL to EL was 1.21 +/- 0.02 (SEM) as compared to 1.31 +/- 0.09 (SEM) for glass tubes using TL and EL of the tube at the same pump flow. This similarity of TL to EL velocity ratios for glass tubes and microvessels may be fortuitous or indicate that comparable flow properties and measurement conditions exist for in vitro and in vivo situations. The major finding of the study is, however, that different velocity correction factors exist for EL and TL measurements when the dual-slit correlation method is used to estimate red cell velocities in tubes of an internal diameter of 40-100 microns at normal hematocrits.

Microcirculatory flow changes during tissue growth

Red blood cell (RBC) velocities, lumen diameters, and volumetric flow rates were determined as functions of position and time in the microvessels of repairing tissue grown in a transparent rabbit ear chamber. By making repeated measurements in the same region, and in many instances, in exactly the same vessel over the entire growth period, we have discerned a distinct microcirculatory flow trend. The flow to a particular region (vessel) increases rapidly after an initial lag time, peaks out, and eventually decays to a steady-state level. This behavior is analogous to the vessel density pattern previously observed in this laboratory, but is delayed in time 1 to 2 weeks. These observations are consistent with mechanical, biochemical, and physiological mechanisms underlying neovascularization, tissue growth, and blood flow regulation, and provide a quantitative understanding of these processes.

Autoregulation of blood flow within individual arterioles in the rat cremaster muscle

Autoregulatory responses to alterations in arterial or venous perfusion pressure were determined for individual arterioles within the rat cremaster muscle. The cremaster muscle of pentobarbital anesthetized rats (50 mg/kg, ip) was surgically exposed and maintained in a controlled tissue bath for visualization by in vitro television microscopy. Cremaster bath PO2 was controlled at either a high (approximately 70 mm Hg) or low (approximately 19 mm Hg) level. Inside diameter and red blood cell velocity were measured for individual first (1A), second (2A), or third (3A) branching order arterioles, and instantaneous blood flows within each arteriole were calculated. To measure the autoregulatory responses, we decreased arterial perfusion pressure to the microvascular bed by gradually occluding the sacral aorta. Significant autoregulation was observed in all orders of vessels, but, in general, autoregulation was more pronounced at all vessel levels when bath PO2 was low, and the autoregulatory gain was greater for the smaller vessels compared to the larger vessels. Elevation of venous pressure within the vascular bed by gradual occlusion of the inferior vena cava led to a significant vasoconstriction of the smaller vessels, suggesting that a significant myogenic component was present. The vasoconstriction response to elevated venous pressure was more pronounced when bath PO2 was high. Our data are not consistent with a purely myogenic or purely metabolic mechanism, but suggest that both mechanisms are simultaneously contributing to the local vascular regulation.

Dimethyl sulfoxide (DMSO) and glycerol, hydroxyl radical scavengers, impair platelet aggregation within and eliminate the accompanying vasodilation of, injured mouse pial arterioles

The hydroxyl radical scavengers dimethyl sulfoxide (DMSO) and glycerol were effective inhibitors of platelet aggregation in an in vivo mouse model of pial arteriolar injury. Aggregability was expressed in terms of the time required for a noxious stimulus (light + dye) to initiate aggregation. These drugs, given 1 hour before the injury, also eliminated the dilation which accompanied the damage. The same drugs failed to influence the constriction which accompanied an identical injury to mouse mesenteric arterioles, but again impaired platelet aggregation in the damaged mesenteric vessel. The data support the concept recently introduced by others, that, in the brain, hydroxyl radicals may mediate vascular damage and/or dilation accompanying the damage. The data also support the concept platelet aggregation may be stimulated, directly or indirectly, by hydroxyl radical. The effects of DMSO and glycerol in this study, irrespective of the molecular basis for the effects, may be relevant to the reported therapeutic benefit of these agents in cerebrovascular disease.

Analogs of phosphatidylcholine: alpha-adrenergic antagonists from the renal medulla

Antihypertensive polar renomedullary lipid (APRL), a conglomerate of 1-0-alkyl-2-acetoyl-glycero-3-phosphocholine analogs, ws tested in 4- to 6-week-old spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats using microcirculatory techniques. APRL (0.5 ug/ml), when added to the solution bathing the cremaster muscle, caused significant changes in the diameter, red blood cell velocity, and blood flow in both groups of rats, for arterioles and venules. Arteriolar changes in diameter were significantly greater (p less than 0.05) in SHR than in WKY. Micropipette application of APRL indicated a dose-dependent response for arterioles and venules in both groups. Moreover, the potent nature of this compound was demonstrated. Relative potency of APRL given intravenously was tested in 10- to 12-week-old SHR and WKY. The response curve was shifted significantly to the left for SHR (p less than 0.01). APRL interaction with known controllers of blood flow was tested in SHR. Blockade of cholinergic, beta-adrenergic, or histaminergic receptors did not inhibit APRL action. blockade of prostaglandin or bradykinin synthesis did not prevent depression of blood pressure by APRL. APRL (40 ug/kg) inhibited (p less than 0.001) the pressor response to norepinephrine (1-10 ug/kg) but not to angiotensin II (4 ug/kg). The present study provides direct evidence that APRL is a vasodilator with increased potency in SHR hypertension. The acute vascular response may be mediated by alpha-adrenergic antagonism.

Blood flow velocity in pulmonary microvessels of bullfrog

Flow velocity in the pulmonary microvessels of the exposed lung of bullfrogs was measured by means of a laser Doppler microscope of an oblique backward mode, together with a signal-analyzing system having a time sharing circuit triggered by the R-wave of the ECG. By these means, measurements of the changes of flow velocity contour in the cardiac cycle were made. Flow velocity was clearly pulsatile in response to cardiac cycles in all microvessels including capillaries. Flow velocities in the arteriole and venule consistently decreased for a short period after the R-wave (84 +/- 33 msec (mean +/- SD) in the arteriole and 130 +/- 31 msec in the venule, respectively) and rapidly increased up to a maximicronm value. The mean flow velocities in arterioles (diameter 50 +/- 17 micron) and venules (39 +/- 9 micronm) were 2.29 +/- 0.32 and 2.30 +/- 0.27 mm/sec. The amplitudes of pulsatile flow in these vessels were 0.83 +/- 0.31 and 0.63 +/- 0.16 mm/sec, respectively. In the capillary the times from the R-wave to the minimicronm and maximum values were variable. In some cases the velocity gradually increased without first decreasing and the increase sharply accelerated a certain time after the R-wave. The mean velocity in the pulmonary capillary and the amplitude of the pulsatile flow ere 1.78 +/- 0.31 and 0.37 +/- 0.12 mm/sec, resepctively. The ratios of the pulsatile amplitude to the mean velocity in the pulmonary capillary, venule and arteriole averaged 0.21, and 0.36, respectively.

The distribution of blood rheological parameters in the microvasculature of cat mesentery

In vivo studies of the rheological behavior of blood in the microcirculation were conducted by direct in situ measurements in cat mesentery. Upstream to downstream pressure drops were measured in unbranched arterioles, capillaries, and venules, with diameters from 7 to 58 micrometer. Simultaneous measurements of red cell velocity and vessel geometry facilitated computation of bulk velocity, pressure gradient, apparent viscosity, wall shear stress, and resistance. Arteriovenous distributions of these parameters revealed the following. Maximum pressure gradient (0.015 cm H20/micrometer) occurs in the true capillaries (7 micrometer in diameter); intravascular wall shear stress averaged 47.1 dynes/cm2 in arterioles and 29.0 dynes/cm2 in venules. Extreme values as great as 200 dynes/cm2 were observed in a few shunting arterioles. Apparent viscosity averaged 3.59 cP in arterioles, 5.15 cP in venules, and 4.22 cP overall. Intravascular resistance per unit length of microvessel varied with luminal diameter as a power law function with exponents of -4.04 for arterioles, -3.94 for venules, and -3.99 for all vessels combined. This apparent maintenance of Poiseuille's law is attributed to the opposing processes of hematocrit reduction and decreasing shear rate as blood is dispersed in successive arteriolar segments, and the converse action of these processes in the venous confluences which lessen the extent of network variations in apparent viscosity. Reductions in bulk velocity from the normal flow state to below 0.5 mm/sec resulted in increases in apparent viscosity by a factor of 2 to 10, which are attributed primarily to obstruction of the lumen by leukocyte-endothelium adhesion.

Quantitative studies of microcirculatory structure and function. III. Microvascular hemodynamics of cat mesentery and rabbit omentum

We made simultaneous measurements of intravascular pressure and red blood cell velocity for vessels which make up the modular configuration of microvascular networks in mesentery and omentum. An analysis of these variables and the computed volumetric flow rates is presented for arterioles which had a maximum diameter of 56 micrometer through the "true capillaries" (typically 7 micrometer for mesentery and 8 micrometer for omentum) to 56-micrometer venules. The spatial variance of pressure and flow is related to topographical features of each network. Vascularization statistics for each network are presented and demonstrate a unique ratio of potential microvascular exchange area to module planar area, with values of 0.71 +/- 0.22 (SD) for omentum and 0.19 "/- 0.03 (SD) for mesentery. Analysis of the volumetric flow rate for each module demonstrates a linear relationship to the planar area of tissue serviced by each modular network. In situ perfusion rates of 1180 ml/min per 100 g and 105 ml/min per 100 g were determined for omentum and mesentery, respectively. The hemodynamic resistance of the omental and mesenteric circuitry was evaluated, and in the case of the omentum, found to be inversely proportional to the planar area of the module. The arterial to venous distribution of pressure and flow for the mosaic of contiguous modules in omentum and mesentery is described and related to the deployment of parallel and serial microvessels of each network.

Comparison of arteriolar blood flow in the hamster cheek pouch at two different oxygen tensions

Red cell velocity and diameter were measured in arterioles of the hamster cheek pouch microvasculature. The cheek pouch tissue was exposed to two different oxygen tensions, PO2 less than 30 mmHg and PO2 greater than 150 mmHg. Blood flow was calculated from velocity and diameter measurements and was found to be significantly reduced (44%) when cheek pouch was exposed to room oxygen superfusant. This reduction was the combined result of a significant decrease in both diameter (17%) and red cell velocity (17%) as compared to the measurements during the initial low oxygen period.

A method for determining segmental resistances in the microcirculation from pressure-flow measurements

On the basis of an electrical analog, open circuit impedance functions were used to analyze the microcirculation. No specific structure need be assumed except a two-port, two-terminal network in which the major artery and vein supplying the tissue represent the input port and the two ends of the microvessel under study are the output port. The open circuit measurements were made by occluding microvessels in the exteriorized omentum of anesthetized rabbits. The pressure upstream and downstream to the occlusion defines the source pressure of a Thévenin's equivalent circuit. The equivalent resistance value was calculated by plotting the flow through a given microvessel against the pressure developed during a gradual occlusion. The changes in pressure vs. the changes in flow during a progressive occlusion were found to be linearly related. The Thévenin's equivalent resistance was maximum downstream to an occluded artery and upstream to the occluded vein. Within the capillary network, source pressures consistently were within a narrow range. Topically applied norepinephrine resulted in marked changes in source resistance and no changes in source pressures. Threshold doses of norepinephrine given intravenously resulted in changes in source pressures, but minimal changes in source resistance, even though a substantial change in vascular resistance was indicated when calculated on the basis of arterial pressure minus micropressure divided by microvessel flow. The present method defines the functional characteristics of the distributing vessels in terms of two pressures and two equivalent resistances and is relatively easy to perform. The technique can be used to determine the vascular components involved in the response to particular stimuli.

Hepatic sinusoidal responses to intraportal injections of phenylephrine and isoprenaline in the rat

1. Specific alpha- and beta-adrenoreceptor agonists, phenylephrine and isoprenaline, were injected intraportally into the intact rat liver under direct microscopic observation by an in vivo transillumination technique. 2. The diameter of a hepatic sinusoid and the intra-sinusoidal erythrocyte velocity were quantitatively measured, and the sinusoidal volume flow was calculated from these two parameters. 3. Results show that phenylephrine causes a sinusoidal constriction and an increased sinusoidal blood flow, whereas isoprenaline causes the opposite effects on the sinusoids. 4. All the sinusoidal responses to phenylephrine and isoprenaline were dose-dependent and were possibly related to the direct effect of these drugs on the sinusoids.

Method for simultaneous determination of red cell and plasma flow velocity in vitro and in vivo

A method is described which can be used to simultaneously determine the flow velocity of plasma and of red blood cells in small glass tubes in vitro or in living microvessels of the microcirculation. The principle of dual slit photometry is applied to the measurement of plasma flow by determining the passage time of a dye bolus across two photodetectors separated by a variable distance. Measurements performed both in vitro and in vivo indicate a significant difference (up to 85%) between cellular and plasmatic flow velocity.