Explanation

Blood vessels are like pipes through which blood flows. The flow of blood through these vessels is influenced by several factors, one of which is the radius (the width) of the vessel. When the radius of a blood vessel decreases, it becomes narrower, just like a thin straw compared to a wider one.

Now, there's a principle in fluid dynamics called Poiseuille's law, which tells us how changes in the radius of a tube affect the flow of fluid through it. According to this law, the flow is proportional to the fourth power of the radius of the tube. This means that even a small change in the radius can have a huge impact on the flow.

In this scenario, the radius of the blood vessel decreases by 30%. So, if the original radius was 100%, now it's only 70% of what it was. Since flow depends on the radius to the fourth power, we calculate the new flow as `(0.70)^4`.

After doing the calculation, we find that the new flow is approximately 24% of the original flow. This dramatic decrease shows how sensitive the flow is to changes in the radius of the vessel. So, when the radius of a blood vessel decreases by 30%, the flow through that vessel becomes only about 24% of what it was originally.