Shear rate and orientation of erythrocytes in pulmonary microvessels of bullfrogs.

Wall shear rates in arterioles and capillaries in the surface of exposed bullfrog lung were estimated to be 436 and 975 sec-1, respectively, at the intra-lung pressure at which a maximum flow velocity was observed. These high shear rates will probably permit an orientation of erythrocytes in a high degree. An orientation of erythrocytes was confirmed at a wall shear rate smaller than 16 sec-1 by means of a microscope connected with a video camera system. Erythrocytes kept their long axis along the direction of the overall blood flow, even when the blood flow transiently stopped flowing during the diastolic phase. The orientation of erythrocytes in such a high degree will be effective to reduce the blood flow resistance in pulmonary microvessels.