Erythrocyte orientation, deformation, and axial accumulation cause differences in resistance between flowing and resting blood. Through theoretical calculations and experimental measurements, we studied the effects of these factors and of sinusoidal and pulsatile flow on blood resistivity. The effect of axial accumulation is least significant. Frequency characteristics of blood resistance under sinusoidal and pulsatile flow showed that erythrocytes cannot orient in response to rapid flow changes of a few pulses per second. From dog aorta investigations, we conclude that the orientation effect on impedance plethysmography pulse waveforms nearly equals that of blood vessel diameter change, and that waveform analysis must consider both effects.