Hematocrit Distribution Research Articles

Transmural hematocrit gradient in left ventricular myocardia of rats

The transmural distribution of tissue hematocrit was determined in rat left ventricular myocardium. Intramyocardial distribution volumes of 99Tc-labeled red blood cells and 131I-labeled albumin were measured in hearts arrested in a relaxed state by KCl or in a contracted state by BaCl2. Hearts were frozen in situ immediately after arrest by bathing in a mixture of liquid nitrogen and isopentane. After excision of the heart, a parallelepipedic portion was cut in a transverse section of the left ventricular free wall and subjected to en face serial sectioning through the thickness of the wall from the epicardium to the endocardium. 99Tc and 131I activity allowed determination of red blood cell and plasma volume and consequently of tissue hematocrit in each section. In both the relaxed and the contracted states of the hearts, a significant decreasing gradient of tissue hematocrit was found from the outer to the inner layers of myocardium. Tissue hematocrit in the subendocardial layers was approximately 70% of its value in the subepicardial layers.

A two-fluid model for hematocrit distribution in microvascular networks.

A new theoretical network model for evaluating discharge hematocrits, explicitly based on plasma skimming at branches, is introduced. The particular network geometry chosen simulates bat wing microvasculature. Blood in vessels is approximated to be a two fluid with red cell suspension as the core and a plasma layer surrounding it. The plasma layer width depends on hematocrit, which leads to nonlinear hydrodynamic equations solved by iteration. Discharge hematocrit distributions are calculated by a computer for five generations of vessels. Dispersion of hematocrit values was found to be correlated to plasma skimming at branches. Contrary to previous suggestions, plasma skimming did not result in lowered mean hematocrit towards the capillaries. Network structure was found to be an important factor affecting the hematocrits. Mean discharge hematocrit remained steady against changes in vessel dimensions, capillary resistances, red cell concentration in plasma layer, and shape of the separation surface defining the streamlines entering the side branch. This high stable mean hematocrit is based on symmetry of the model network. Enhanced asymmetry tended to lower the hematocrit.

Computer simulation of cerebral microhemodynamics.

Microvascular network hemodynamics was simulated by computer in an anatomically reconstructed cerebral microvascular network. A video microscope system was used for three-dimensional mapping of the vessel network in the rat brain cortex. The complete topology, length and mean diameter of the microvessels were determined. The distribution of blood flow and red cell flux in the network was calculated based on vessel resistance estimated from geometrical data and a rheological model of blood. This model described apparent relative blood viscosity as a function of vessel diameter and local discharge hematocrit. The calculations predicted highly heterogeneous cell flux distribution at any feed hematocrit between 10 and 40 percent. The frequency distribution of microvessel hematocrit was bimodal and included values exceeding the feed hematocrit value. A probabilistic simulation of cell transit resulted in transit time distributions which agree with experimental findings. The most probable transit time and capillary path length and 4s and 300 microns, respectively.

Haematocrit distribution in rabbit tenuissimus muscle.

Low values of mean capillary haematocrit have been reported in many tissues including skeletal muscle. The present study was undertaken to analyse haematocrit distribution in the transverse and terminal arterioles, capillaries and venules of the rabbit tenuissimus muscle preparation. Tube haematocrit, i.e. the volume fraction of red cells, in muscle capillaries (n = 85) was found to be 39% of systematic haematocrit Hsys. In part, this haematocrit reduction is due to the Fahraeus effect. Corresponding capillary discharge haematocrit HD was 56% of Hsys. Tenuissimus muscle capillaries are fed by terminal arterioles originating from transverse arterioles. The latter extend into and supply adjacent connective tissue septa in addition to the muscle tissue proper. In transverse arterioles leaving the muscle to enter the connective tissue, HD was found to be 127% of Hsys (n = 18), and in collecting venules at the muscle edge HD was 129% of Hsys (n = 18). These findings indicate that the connective tissue microcirculation represents a functional red-cell shunt in resting tenuissimus muscle. Since only about 20% of the inflow to the preparation passes through the connective tissue, this shunting is not sufficient to satisfy conservation of red-cell mass. In addition, it is likely that the observed low capillary haematocrit is in part due to a positive correlation between blood-flow velocity and HD in capillaries originating from individual terminal arterioles. This phenomenon is called the network Fahraeus effect.

Microvascular hematocrit and red cell flux in rat cremaster muscle.

The arteriovenous distributions of volumetric flow (Q), microvessel hematocrit (Hctmicro), and estimates of red cell volumetric flux (QRBC) were obtained under control conditions in rat cremaster muscle. The results demonstrate a monotonic fall in the ratio of Hctmicro/Hctsystemic from 0.86 in 70-microns arterioles to 0.48 in capillaries followed by a subsequent rise to 0.79 in 98-microns venules. To assess the roles of Hctmicro and Q in red blood cell delivery following a period of reduced oxygen transport, tissue ischemia was produced by occluding the first order arteriole. During the occlusion, arteriolar and large venular hematocrits fell 15-30%, whereas small venular hematocrits increased 24%. After release of the occlusion, a reactive hyperemia ensued with Q, QRBC, and QHctmicro increasing significantly above control values in arterioles, capillaries, and venules. All Hctmicro returned to their control values within 10 s following resumption of flow. Based on the relationship between blood viscosity and Hctmicro, at low shear rate, these transient alterations in Hctmicro were estimated to have a profound effect on blood viscosity, and hence the resistance to blood flow. Such changes may affect recovery from an ischemic episode, although not adversely affecting the oxygen-carrying capacity of blood and convective transport of oxygen.

Generalization of the Fahraeus principle for microvessel networks.

Microvessel hematocrits and diameters were determined in each vessel segment between bifurcations of three complete microvascular networks in rat mesentery. Classification of the segments as arteriolar, venular, or arteriovenular (av) was based on flow direction at branch points. Photographic and videomicroscopic mapping was used to obtain quantitative information on the architecture and topology of the networks. This topological information allowed the analysis of hematocrit distribution within a series of consecutive-flow cross sections, each of which carried the total flow through the network. The observed reduction of mean hematocrit in the more peripheral cross sections is explained by the presence of a "vessel" and a "network" Fahraeus effect. The vessel Fahraeus effect results from velocity difference between red cells and blood within the individual vessel segments due to the existing velocity and cell concentration profiles. The network Fahraeus effect is based on the velocity difference of red cells and blood caused by velocity and hematocrit heterogeneity between the vessels constituting any of the complete-flow cross sections. The network Fahraeus effect is found to account for approximately 20% of the total hematocrit reduction and increases toward the most distal cross sections.

Effect of dispersion of vessel diameters and lengths in stochastic networks: II. Modeling of microvascular hematocrit distribution

A microvascular network model described in the preceding paper ( B. Dawant, M. Levin, and A. s. Popel, 1986, Microvasc. Res. 31, 203–222) featuring random distribution of vessel diameters and lengths, is extended to calculate the distributions of red blood cell flux and discharge hematocrit throughout the network. A relationship between red blood cell fluxes and blood flow rates at vascular bifurcations is incorporated into the model, and the effect of the form of this relationship on RBC distribution is investigated. The mean capillary discharge hematocrit is sensitive to the variation of parameters describing this relationship; thus detailed experimental information is required for bifurcations of different sizes and types.

Regional lung water and hematocrit determined by positron emission tomography.

We have measured with positron emission tomography (PET) the regional distribution of extravascular lung water (EVLW) and hematocrit (HctL) in normal supine dogs. H2(15)O and C15O were used as total lung water (TLW) and intravascular water (IVW) compartment labels, respectively. An additional plasma volume label (68Ga-transferrin) was used to determine regional HctL. EVLW was calculated as the difference between TLW and IVW. In 13 dogs, EVLW was relatively constant along a gravity-dependent vertical gradient, although values in the most anterior regions were statistically less (P less than 0.05) than those in more posterior ones. The average value for EVLW (13 dogs) was 14.4 +/- 2.5 ml H2O/100 ml lung. When EVLW was compared with IVW on a regional basis, the EVLW/IVW ratio decreased significantly in a gravity-dependent direction from 1.95 +/- 0.28 to 0.88 +/- 0.18. In 7 dogs, no significant difference between HctL and systemic hematocrit (average ratio 1.01 +/- 0.08) was found nor was any significant variation of HctL within the lung detected. Thus, in contrast to gravimetric techniques, a hematocrit correction does not appear to be necessary when regional EVLW is studied by PET.

Nonuniform red cell distribution in 20 to 100 μm bifurcations

Several potentially important parameters influencing the disproportionate distribution of red cell flux and blood flow at a bifurcation are examined. These include parent vessel hematocrit, vessel diameter, suspending medium, cell distensiblity, parent vessel blood flow rate, and local geometry. Measurements were performed on 20 to 100 μm bifurcations, fabricated such that all vessels of a given bifurcation have the same diameter. Suspensions of human red cells, hardened red cells, and mixtures of each in albumin-saline, Dextran 75, or plasma were flowed through the bifurcations and determinations of flow rates and discharge hematocrits were made for each of the channels. For the 20-μm channels, hematocrits were found using videophotometric techniques, and for the larger channels, hematocrits were measured directly from the exit streams. Flow rates for both were measured by meniscus travel downstream in small-diameter glass tubes. Within the limits of the present experiments, three of the variables proved to be of major importance: (i) feed hematocrit, (ii) tube diameter, and (iii) flow-rate distribution. It was clearly demonstrated that red cell flux varies nonlinearly with fractional flow rate. Critical flow rates, at which all or none of the cells entered one of the branches, were found to vary with diameter and hematocrit as has been reported in other studies. The data were analyzed with a theoretical model which assumes that the parent vessel contains a core of uniformly distributed red cells surrounded by a marginal gap of suspending medium; in the parent vessel lumen, the flows to the two daughter branches were assumed to be separated by a chord. The marginal gap widths and tube hematocrits deduced from the data with this model are of reasonable magnitudes.

Microphotometric determination of hematocrit in small vessels.

Evaluation and calibration of a microphotometric method for hematocrit determination in microvessels of transilluminated tissues is described. This method is based on the relation of the optical density (OD) of a microvessel to its hematocrit (Hct). The following requirements of the microphotometric system appear essential: narrow-band monochromatic light source with efficient false light suppression, high numerical aperture of the objective and low numerical aperture of the condensor. We used a video system to measure the intensities of incident (Io) and transmitted (I) light. For converting of Io and I into OD values, correcting procedures were evaluated to eliminate the influence of glare, shading, and fading. The calibration procedure was performed with glass tubes of inner diameter (ID) between 13 and 68 microns perfused with red cell suspensions. A function was fitted to the data, correlating OD to ID and Hct. The standard deviation of the original data from this function is +/- 0.02 units of fractional hematocrit. The presented method allows the continuous determination of the hematocrit in a microvessel as well as the off-line evaluation of the hematocrit distribution within a microvessel network.

Distribution of flow and red cell flux in the microcirculation

The distribution of blood cells in the vascular system is not homogeneous, red cell concentration in the small vessel compartment being significantly lower than in the large vessel compartment. This is to some extent due to the difference in travelling speed between cells and plasma in the microcirculation (Fahraeus effect). However, even in the terminal vessel network haematocrit varies considerably as a result of plasma skimming phenomena. Thus, local (capillary) haematocrit is a function of flow rate distribution at bifurcations. Red cell concentration in the majority of capillaries is substantially lower than in any other vessel category, while a small number of capillaries carry an increased haematocrit, thereby satisfying mass balance. The haematocrit distribution as well as the average level of capillary haematocrit is a function of small vessel volume which in turn depends on pre-capillary resistance: vasodilation leads to increased capillary haematocrit and vice versa. Thus, one of the physiological functions of the resistance vessels is to determine the level of microvascular haematocrit. Alteration of red cell microrheology leads to a reduction of capillary O2-transport capacity mainly by reducing the haematocrit in nutritive capillaries.

In vivo measurements of “apparent viscosity” and microvessel hematocrit in the mesentery of the cat

The arteriovenous (A-V) distribution of microvessel hematocrit ( H micro) was determined throughout successive microvascular divisions in cat mesentery from in vivo measurements of optical density. In vitro correlations of optical density and tube hematocrit in small-bore glass tubes permitted the computation of in vivo values of H micro for luminal diameters ranging from 20 to 70 μm. For smaller-size vessels, H micro was determined by microocclusion and red cell counting. The results demonstrate a monotonic fall in the ratio of H micro H systemic from 0.80 in the 70-μm arterioles to a minimum of 0.21 in the immediate postcapillaries (10 μm diameter) followed by a subsequent monotonic rise to 0.95 in the 70-μm venules. Conservation of red cell flux throughout the mesenteric network was partially demonstrated upon applying previously established in vitro relationships between discharge and tube hematocrits, the resulting disparity being attributed to the rheological behavior of blood and possible A-V shunting of red cells. Simultaneous measurements of pressure drop and red cell velocity in unbranched arterioles during systemic hemodilution facilitated a comparison of in vivo and in vitro (cone-plate viscometer) apparent viscosities (η). No significant differences between the two approaches were found for arteriole diameters ranging from 24 to 47 μm in the absence of leukocyte-endothelium adhesion, with 0 < H micro < 36% and vivo wall shear rates above 500 sec −1. In situ measurements of η were found to increase 74% due to leukocyte adhesion during sustained flow reductions, in one illustrative case. Values of η computed from the A-V distribution of H micro and the regression of η vs H micro (established without leukocyte adhesion), in comparison with previous measurements of the A-V distribution of η with leukocyte adhesion present, suggest that leukocyte adhesion may result in overall increases in η ranging from 100 to 200%, throughout the mesenteric network.

Regional distribution of extravascular water and hematocrit in the lung.

Regional distribution of extravascular water and hematocrit in the lung.

Effect of blood rheology on hematocrit distribution in pulmonary sheet flow.

Using a physical model of convenient macroscopic scale with blood simulated by gelatin pellets in a silicone fluid, the red cell distribution in the pulmonary alveoli was studied. Information pertaining to the flow characteristics of blood at the bif

Red cell values on the first postnatal day during the last 16 weeks of gestation

The hemoglobin concentration, red cell count, hematocrit, reticulocyte count, and red cell size distribution were determined on skin prick blood obtained on the first postnatal day from infants born at various stages of gestation, from week 24 to term. During this period the hemoglobin concentration and hematocrit remained constant, around 19 gm/100 ml and 60%, respectively. The mean corpuscular volume decreased progressively, with a corresponding rise in the red cell count. The reticulocyte count decreased progressively from 9.6% to 3.7% during the period of observation. The red cell size distribution (Price Jones) curve was markedly shifted to the macrocytic side at 24-25 weeks. Thereafter, cells larger than 102 mu3 decreased gradually, while the percentage of smaller cells increased. These observations are not consistent with the hypothesis of a progressive decrease in the supply of oxygen to the fetus during the later stages of gestation, to which the fetus adapts by increasing its hemoglobin concentration.

Stochastic flow in capillary blood vessels

The features of the dynamics of capillary blood flow are analyzed and compared with those of the large blood vessels. The physical causes of the constantly varying unsteady flow are attributed to the random statistical distribution of red blood cell sizes, blood vessel geometry, and the multiloop system. The influence of the capillary system topology and compliance are discussed. Among other things, it is shown theoretically that (1), at a branching point of a capillary blood vessel the branch with a faster stream gets most of the red blood cells, thus explaining the extreme nonuniformity in hematocrit distribution in the capillaries, and (2), in a contracting muscle bundle in which capillaries are parallel to the muscle fibers, the percentage reduction of resistance to blood flow is three times the percentage reduction in length of the muscle bundle if active control by sphincter is absent.

Distribution of cardiac output and organ blood content in anemic and polycythemic rats.

Organ blood content, tissue hematocrit, and regional distribution of cardiac output were measured in anemic and polycythemic rats. An increase in the blood content was found in the gastrocnemius muscle of anemic animals, together with a decrease in the blood content of the spleen. In polycythemic rats, increases in the blood content of the liver and of the kidney were detected. The blood content of the terminal vessels in the heart muscle yields a good estimate of the capillary density as demonstrated by a comparison with direct histological measurements. Variations in tissue hematocrit in the heart and gastrocnemius muscle were similar to those in venous blood whereas in other organs variations were greater than those in venous blood. Distribution of the cardiac output was changed in both experimental states; the greatest variation was found in the renal fraction (lower in anemic and higher in polycythemic rats). Finally, a possibility for comparing the relative blood flow in animals of different body weights was discussed and a new index was proposed as: percentage of tissue rubidium uptake/percentage of body weight.