||Human Homo sapiens
||P. Butti et al., Technical report: Capillary red blood cell velocity measurements in human nailfold by videodensitometric method, Microvascular Research, Volume 10, Issue 2, September 1975, Pages 220-227 link p.226 top paragraph
||P.226 top paragraph: "In terms of the absolute values of the measured speeds it was noted that the apparent diameter of nailfold capillaries tends to increase progressively and continuously from the arteriolar origin toward the venous end. In effect, diameters at the proximal end of the visible arteriolar capillary limb are of the order of 9-12 µm while in the venous end the apparent lumen is in the 12-18 µm range. This configuration determines that the flow of red cells is comparatively rapid in the arteriolar limb, and slower in the venous limb." P.226 2nd paragraph: "It should be noted that the diameters measured with the television microscopy
technique are those shown by the width of the red cell column, and not the actual capillary lumen, since the endothelial lining is not discernible."