The axial blood velocity (Vax) association with microvessel diameter (D) was studied at 104 different postcapillary venules (4 μm < D < 24 μm) and 30 different precapillary arterioles (6 μm≤D≤12 μm) in the human conjunctiva of normal healthy humans. Venular diameter sizes were classified as “very small” (Group 1, 4.4 μm≤D < 8.9 μm), “small” (Group 2, 8.9 μm≤D < 13.8 μm), “medium” (Group 3, 13.8 μm≤D < 19.1 μm) and “large” (Group 4, 19.1 μm≤D≤23.5). The Spearman’s correlation coefficient (rs) in all 4 venular groups was less than 0.36 and not statistically significant (n = 26, p≥0.08). Similar correlation results were observed for the arteriolar group (rs) ≈ 0) for the peak systolic, the average and the end systolic axial velocities. Vax was significantly (p < 0.001) lower in Group 1 in comparison to that in Group 2 and significantly (p < 0.01) lower in Group 2 in comparison to that in Group 3. However, Vax was not significantly lower in Group 3 in comparison to that in Group 4. Average Vax and standard deviation was 0.48 ± 0.13, 0.64 ± 0.16, 0.82 ± 0.25 and 0.88 ± 0.32 mm/s for Groups 1, 2, 3 and 4 respectively. The above results reinforce the importance of measuring D in microvascular hemodynamics. Higher diameters suggest higher axial velocities but Vax does not change significantly within the limits of each of the aforementioned groups.
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