Flow-dependent rheological properties of blood in capillaries

Variation in axial velocity profile of red cells passing through a single capillary

We have used an analysis of the velocity of individual red cells as the cells pass through a capillary in order to estimate the variability in cross-sectional area of the capillary lumen available for flow along the length of the vessel. The purpose of the study was to determine if there were irregularities of sufficient magnitude and frequency to support Secomb's hypothesis that local constrictions in the capillary lumen could hinder blood flow at low driving pressure, due to the energy required to deform red cells as they pass through the constriction. All capillary segments analyzed to date, in both rat and frog, have shown regions where the velocity of individual cells is consistently faster or slower than that of the mean velocity of all other cells in the same segment. There are approximately two constrictions per 100 microns in the rat and one per 100 microns in the frog. On average these constrictions appear to reduce the cross-sectional area by 30% in the rat and 16% in the frog. These results provide evidence in support of Secomb's hypothesis. In addition, our results from one bifurcation indicate that the capillary lumen increases in cross-sectional area as one moves from the parent vessel to the region of the bifurcation. Downstream of the bifurcation the lumen rapidly decreases in area by 45 to 54%. Thus a red cell must undergo even greater deformation as it passes through a capillary bifurcation than it will in most other sections of the capillary network.(ABSTRACT TRUNCATED AT 250 WORDS)

Capillary lengths and anastomoses in rat hindlimb muscles, studied by Aquablak perfusion during rest versus exercise

This investigation shows that provided an adequate perfusion time of the capillary network is allowed following injection of Aquablak, the presence of arterioles and capillaries having zero or near-zero flow rates can be demonstrated in resting muscle. During hyperemia, "flow recruitment" occurs in these vessels, as indicated by their perfusion with Aquablak. Our observations of Aquablak perfusion in hyperemic muscles show that in medial gastrocnemius, gracilis, and soleus the mean arteriolar-to-venular distances, and also the mean capillary pathlengths, were not dramatically different. What was striking, however, was the fact that capillary pathlengths in soleus were divided into twice the number of segments found in gastrocnemius and gracilis. This suggests the possibility that in oxidative muscles the capillary network may exhibit a much higher degree of branching than in glycolytic muscles. This would increase the area for diffusional exchange between blood and tissue in oxidative compared to glycolytic muscle.

Interaction between bending and tension forces in bilayer membranes

A theoretical analysis is presented of the bending mechanics of a membrane consisting of two tightly-coupled leaflets, each of which shears and bends readily but strongly resists area changes. Structures of this type have been proposed to model biological membranes such as red blood cell membrane. It is shown that when such a membrane is bent, anisotropic components of resultant membrane tension (shear stresses) are induced, even when the tension in each leaflet is isotropic. The induced shear stresses increase as the square of the membrane curvature, and become significant for moderate curvatures (when the radius of curvature is much larger than the distance between the leaflets). This effect has implications for the analysis of shape and deformation of freely suspended and flowing red blood cells.

Evidence for increased perfusion heterogeneity in skeletal muscle during reduced flow

Recently, it has been demonstrated (D. Cousineau, C. P. Rose, D. Lamoureux, and C. A. Goresky, 1983, Cir. Res., 53, 719-730; K. Tyml, 1986, Microvasc. Res., 32, 84-98) that heterogeneity of microvascular flow depends on tissue metabolism. The objective of this study was to examine the possibility that, independent of metabolism, heterogeneity is also a function of flow. Using an intravital video-microscopic approach, we evaluated heterogeneity in the frog sartorius muscle at different flow rates while maintaining the muscle in the same exercised state. The flow was altered via partial aortal clamping. Exercised state was achieved by direct electrical stimulation. Heterogeneity of flow was evaluated in terms of the coefficient of variation (CV = SD/mean) computed from simultaneous measurements of red cell velocities in a capillary network. In addition to velocity analysis, the number of perfused capillaries crossing a 1-mm test line on the video monitor was counted. Among 10 networks from eight muscles, the overall preocclusion hyperemic velocity and CV were 0.42 +/- 0.15 SD mm/sec and 40 +/- 12%, respectively. The overall capillary count was 16.4 +/- 3.0 cap/mm. In all networks, increasing clamping reduced the mean hyperemic velocity and increased CV. For reductions to less than 50% of the preocclusion velocity, CV increased significantly (P less than 0.05), up to 88%. In 6 networks only, increasing clamping reduced capillary count, down to 10.2 cap/mm. This reduction was due to flow stoppages in capillaries situated randomly throughout the network. The data demonstrate for the first time that, for the same exercised state, heterogeneity of velocity depends on the mean velocity.(ABSTRACT TRUNCATED AT 250 WORDS)