Why does the narrowing of the arteries decrease blood ﬂow but increase blood pressure?
If you narrow any tube, the flow of fluid will decrease.
Imagine a rubber tube and think of pinching it. The flow of fluid will slow down. The same occurs in the arteries of the body.
The problem is not as important in the veins as in the arteries.
But since the flow is reduced, how will the body compensate for that? You do need a certain volume of blood flow to the head or you will pass out.
The only way to do this is to somehow increase it by making the heart beat harder and faster.
This will allow the blood to push harder against the tube (vessel) as it does it will expand some. This causes the blood pressure to go up and flow to increase.
This the definition of what we call high BP. "High blood pressure is a common disease in which blood flows through blood vessels (arteries) at higher than normal pressures".