Final answer:
A negative Doppler shift occurs when red blood cells are traveling away from the transducer due to the Doppler Effect.
Step-by-step explanation:
If the red blood cells are traveling away from the transducer, a negative Doppler shift is present. In physics, the Doppler Effect refers to the change in frequency of a wave in relation to an observer who is moving relative to the wave source. In the context of medical ultrasound, a negative Doppler shift occurs when the blood cells are moving away from the probe, causing the returning ultrasound waves to have a lower frequency compared to the emitted waves. Conversely, if the blood cells were moving towards the probe, this would result in a positive Doppler shift.