Final answer:
The Doppler ultrasound probe should be placed at a 45 to 60-degree angle for accurate blood velocity measurements, and blood speed increases at points of constriction due to the Bernoulli effect and to maintain consistent flow.
Step-by-step explanation:
When using a Doppler ultrasound device, the vascular probe should typically be positioned at an angle of 45 to 60 degrees to the surface of the skin, over the vessel being examined. The correct angle is crucial for accurately measuring blood velocity. At or after a partial constriction of an artery, one would expect the blood speed to be greatest at the constriction due to conservation of mass and increased velocity required to maintain flow rate. The two distinct causes of higher resistance in the constriction are increased fluid velocity and the Bernoulli effect, which describes the inverse relationship between velocity and pressure in a flowing fluid.