Final answer:
As vessel radius decreases, resistance increases due to the inverse relationship to the fourth power of the radius, while the flow rate decreases as a result of the increased resistance.
Step-by-step explanation:
When the radius of a vessel decreases, the resistance to blood flow increases. This is because resistance is inversely proportional to the fourth power of the radius of the vessel. A small decrease in the vessel radius can dramatically increase resistance. Consequently, this would mean the blood flow rate would decrease within that vessel due to the greater resistance it encounters.
So, when we consider the effect of a decreasing vessel radius, more than one of the provided options are correct: Resistance increases and flow rate decreases.