Final answer:
In a partially constricted artery, blood speed is greatest at the point of constriction, as the blood must move faster through the narrower region to maintain flow rate. Increased resistance arises from the narrowing that increases friction and the potential onset of turbulence causing chaotic flow patterns.
Step-by-step explanation:
During a Doppler ultrasound examination of a partially constricted artery, one would observe that the blood speed is greatest at the point of constriction. This is because as the cross-sectional area of the artery decreases at the constriction point, the blood must move faster through this narrowed region to maintain the same flow rate according to the principle of continuity where Q = Av (with Q being the flow rate, A the cross-sectional area, and v the velocity).
Increased resistance in the constriction arises from two major factors. One is the physical narrowing of the vessel which increases fluid friction. Another cause is the potential onset of turbulence as fluid speeds increase beyond a critical point which creates chaotic flow patterns and thus, adds to the resistance.