Comparative flow velocity of erythrocytes and leukocytes in feline retinal capillaries.

PURPOSE To study the flow velocity of erythrocytes and leukocytes in the same retinal capillaries in cat eyes. METHODS Blood cells from cats were stained with fluorescent label and reinjected into the animals. An electro-optical device combined with a highly sensitive CCD camera detected the fluorescent signal from the labeled blood cells. The signal was stored on a videotape (VHS-PAL, 25 frames per second) and later analyzed digitally. Twenty leukocytes were identified, each flowing through a separate retinal capillary pathway. The capillary pathways were categorized as simple and complex according to their morphologic complexity. Observations were made on 10 simple and 10 complex capillary pathways. The passages of one leukocyte and 20 erythrocytes in each capillary path were recorded and analyzed. The blood cell velocity ratio was calculated from the number of frames needed for each cell to complete a passage through the capillary pathway (higher velocity = fewer frames). RESULTS The erythrocytes traveled faster than the leukocytes in both the short and long capillary pathways. The frames ratio (mean percent +/- standard deviation) of erythrocyte-leukocyte passage in the same capillary pathway was 84.55% +/- 2.0% in the short ones and 66.21% +/- 7.1% in the complex ones. The main deceleration in leukocytes flow was noted in the looping parts of the complex capillary pathways. CONCLUSIONS The velocity of leukocytes is slower than that of erythrocytes in a given retinal capillary pathway. The structural complexity of a capillary pathway has a greater effect on leukocytic velocity than on erythrocytic velocity.