I see why you would think so, but in blood vessels what is happening is, when a vessel is narrow, blood cells rub against or bounce off the walls and the friction slows them down; the narrower the vessels the higher the proportion of cells that are near the wall. This is what we term resistance of a vessel. And the higher the resistance the lower the flow.(Original post by 123chem)
hmm, i think like this; in a narrow corrider means its more packed, so people will be moving quiker as more people enter the corridor
Edit: also rate of flow is not the same as velocity: rate of flow is more like how many cells go through per unit time, while velocity is speed. Resistance lowers speed. But a decrease in radius alone can decrease the amount of cells that go through a specific point per unit time.