Harold Wayland

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

Abstract The extreme blunting of velocity profiles for human blood, reported from doubleslit photometric measurements, has been shown to be an artifact of the measuring technique. The nature of the data distortion is an averaging over an area much larger than that predicted from analysis of the optics alone. A similar sensing and averaging of events from an area much larger than assumed may account for blunting of velocity profiles measured by other techniques such as laser-doppler. An empirical result showing a simple constant relationship between the volume flow rate and the double-slit centerline velocity makes the volume flow rate measurement by this technique simple and implies that the actual velocity profiles do not change significantly from a parabolic shape over a wide range of U hematocrit, and tube diameter.

Action of histamine on the mesenteric microvasculature.

Abstract The effect of known, reproducible doses of histamine (0.1–100 μg/ml base) on the permeability of the microvasculature of the cat and rat mesentery was examined in vivo using intravital fluorescence microscopy and in fixed sections from identifiable regions of the same tissue using electron microscopy. The percentage of animals in which extravasation of fluorescent serum albumin in response to histamine was observed was positively related to dose, with approximately half the animals responding to 2–5 μg/ml (rat) or 0.1–0.5 μg/ml (cat). Electron microscopy of sections of vessels exhibiting extravasation of fluorescent tracer in vivo revealed numerous gaps ranging in size from 0.5 to 1.0 μm between endothelial cells. Serial sections and computer three-dimensional reconstructions showed that these gaps occurred at endothelial junctions but with irregular geometry and involvement of “cords” of cytoplasm bridging some of the endothelial gaps.

Interstitial diffusion of macromolecules in the rat mesentery

Abstract Apparent diffusion coefficients of fluorescein isothiocyanate (FITC)-labeled dextrans ( M w 3400–41,200) and rat serum albumin (RSA-FITC) were measured in the interstitial space of the rat mesentery. Average ratios of apparent diffusion coefficient to free diffusion coefficient for the dextrans ranged from 0.26 for dextran 3400 to 0.033 for dextran 41,200 and 0.073 for RSA-FITC. These values are lower than those reported for loose connective tissue (umbilical cord) but greater than those from dense connective tissue (cartilage). The apparent diffusion coefficient for RSA-FITC was approximately one-half the apparent diffusion coefficient of FITC-dextran having the same free diffusion coefficient (and thus the same Stokes radius). Mathematical models describing solute diffusion in gels or glycosaminoglycan solutions as a function of solute size did not give a good fit to the data but indicated that the presence of a relatively dense interstitial matrix is required in order to explain the data.

Macromolecular transport in the cat mesentery

Abstract Movement of macromolecules from the bloodstream across microvascular walls, into and through the surrounding tissue was studied in the cat mesentery with intravital fluorescence microscopy. Rapid passage of small molecules, such as unconjugated fluoroscein isothiocyanate (FITC), MW 389, and FITC-Dextran, MW = 3400, was found along the entire length of most microvessels, although more prominently on venous capillaries and venules. For species with MW 19,000 or higher, a distinctly spotty pattern of movement out of the microvessels was seen, mostly in localized areas on venous capillaries or venules. These localized regions may represent the “large pore” regions frequently postulated. Diffusion coefficients for movement within the interstitial space were calculated for dextrans as well as for bovine serum albumin (BSA), using a one-dimensional diffusion equation with a time dependent boundary condition at the origin. The calculated diffusion coefficients for dextrans of MW 41,200 and below, and for BSA (MW 69,000) are, within experimental error, the same as the free diffusion coefficients for dextrans of the same molecular weight in water. Dextrans with MW = 152,700 and MW = 393,900 showed smaller diffusion coefficients in the tissue than in water, consistent with increased volume exclusion for larger molecules. The calculated diffusion coefficient for BSA is only two-thirds of that in water. This may possibly be explained by the negative charge on the BSA.

On the Mechanical Behavior of Elastic Animal Tissue

In this paper there is introduced a strain energy function which describes a class of materials that includes elastic animal tissue as well as other highly distensible materials. The functional form of this representation is general enough to suggest other forms that may encompass additional classes of material. The theoretical stress‐strain curves are shown to correlate well with experimental data obtained upon loading (as distinguished from unloading) different animal tissues such as frog’s striated muscle, human papillary muscle, and cat’s and rabbit’s mesentery, as well as synthetic materials such as latex rubber. There is also developed a thinness theory which can be used to calculate the deformation field in the case of applied plane stress for the more complicated geometry of a wedge‐shaped (arcuate) specimen. This thinness theory has been applied to a fan‐shaped specimen with tangential and radial loading.

Velocity profiles of human blood at normal and reduced hematocrit in glass tubes up to 130 μ diameter

Abstract Using a photometric method, measurements of red blood cell velocities were performed on human blood flowing through vertical glass tubes at a constant rate. Velocity profiles were obtained by successive measurements at different radial positions across the tube diameter. Measurements were performed over a range of tube diameters between 30 μ and 130 μ at flow rates corresponding to U - values between 1.0 and 300 diameters/second. The velocity profiles were found to be markedly non-parabolic; the deviation from a parabola was largest in the smallest tube, at low flow rates, and at high hematocrits. The effect of the flow rate on the shape of the profile was more pronounced in small tubes, pointing to differences in the flow regime. Predictions by a theoretical model (Casson) of the effect of hematocrit and flow rate on the shape of the velocity profiles in a 130 μ tube showed good correspondence with the experimental results if a cell-free plasma layer at the tube wall of less than 1 μ width was assumed. The results were related to shear-dependent particle-particle interaction and deformation of red cells.

Erythrocyte flow velocities in mesenteric microvessels of the cat

Abstract Using the photometric dual slit technique, measurements of erythrocyte flow velocities were performed in vessels of the cats mesenteric microvasculature. Mean centerline velocities ranged between 31.7 and 1.0 mm/sec in arterioles (up to 60 μ diameter), 11.1 and 0.5 mm/sec in venules of the same dimensions. Capillary flow velocities of 1.7 mm/sec and less were measured. Flow velocities in all vessels (arterial and venous) with the exception of capillaries were found to be pulsatile. Mean flow velocities were rather constant over longer periods of time (minutes), possibly indicating a depression of normal vasomotion due to the anesthesia. In an individual vascular loop, a good correlation between flow velocity and vessel diameter was observed. This correlation was much less evident when measurements were performed in several segments of the mesentery. It is concluded that this reflects the regional differences in blood flow as a result of local regulatory phenomena.

MICROCIRCULATION IN PANCREATIC FUNCTION

The pancreas is involved in two major bodily functions: production of hormones involved in the control of carbohydrate metabolism and the production of enzymes essential to digestion. Pancreatic function is mediated by both neurological and humoral control. The major pathway for humoral control is through the circulatory system, the level of action being in the microcirculation. This introductory paper explores the need for a deeper understanding of the dynamic morphology, i.e. the actual flow patterns in the microcirculation, as a function of the physiological state and demand to complement the careful ultrastructural mapping of the microvasculature. The current state of knowledge in this field is reviewed as a basis for identifying important areas of knowledge and ignorance, and some suggestions are made as to possible procedures for further experimental studies, particularly in the microscopic observation of the dynamics of the microcirculation with special emphasis on the need for transport studies in both directions across the microvascular wall. Microsc. Res. Tech. 37:418–433, 1997.

Photosensor methods of flow measurement in the microcirculation.

Abstract The basic photometric method of Wayland and Johnson for measuring erythrocyte velocity has been extended by several research workers to microvessels ranging from capillaries to venules and arterioles as large as 130 μm. Consideration is given to the use of the erythrocyte as a tracer for measuring erythrocyte flux, erythrocyte residence time, plasma flux, and plasma residence time in vessels of various sizes. Various methods of analysis such as direct or electronic time delay measurement, on-line cross correlation, and frequency analysis are being employed. It is concluded that: (1) the erythrocyte is a valid tracer of local velocity; (2) erythrocyte velocity is readily measurable in capillaries by the two-slit photometric method; (3) erythrocyte flux can be measured in capillaries with a single photometric slit, but frequency analysis of single-slit data requires in situ calibration for determining the absolute velocity; (4) plasma flux can be approximated in capillaries once erythrocyte velocity and flux are known; (5) volume flow of blood in larger microvessels can be deduced from the two-slit photometric data taken at the centerline of the vessel; (6) further refinement is necessary to measure minor blunting of the velocity profile in microvessels; (7) methods of estimating hematocrit in these larger microvessels need to be developed.

Streaming Birefringence of Rigid Macromolecules in General Two‐Dimensional Laminar Flow

Quantitative expressions for the direction of the angle of isocline and for the amount of birefringence due to a dilute solution of rigid macromolecules in a general two‐dimensional laminar flow are derived. If E is the principal strain rate and Λ0 is the angle between the streamline direction and the direction of the principal strain rate axis, the angle of isocline, measured from the first principal strain rate axis is χ=−(Esin2Λ0/6D){1−(E2/27D2)[sin22Λ0+(24b2/35)]+···], where b=[(a12—a22) / (a12+a22)] is a shape factor for an ellipsoid of revolution of semimajor axis a1 and semiminor axis a2 and D is the rotary diffusion constant for this ellipsoid. The amount of birefringence is Δn=(4π/15)(cGEb/nD) {1−(E2/18D2)[sin22Λ0+(6b2/35)]+···}, where n is the mean index of refraction of the solution, c the volume concentration of the macromolecules, and G=g1—g2 is the optical anisotropy of the ellipsoids.It is seen that if the principal strain rate is not at 45° to the streamline at the point of observation, th...