Final answer:
Doppler ultrasonography is used to measure blood speed, not blood pressure. Blood speed is greatest after a partial constriction of an artery. The higher resistance in the constriction is due to increased friction and turbulence.
Step-by-step explanation:
Doppler ultrasonography is not used to measure blood pressure but rather to measure the speed of blood flow in the body. When considering a partial constriction of an artery, we would expect the blood speed to be greatest immediately after the constriction due to the continuity equation and Bernoulli's principle, which explain the behavior of a fluid flowing through a narrowing conduit. The two distinct causes of higher resistance in the constriction are increased frictional resistance due to the narrowing passageway and turbulence that occurs when the speed of the fluid exceeds a certain threshold.