Final answer:
As the diameter of a blood vessel increases, the flow of blood through the vessel also increases due to lower resistance. Conversely, as the diameter decreases, the flow of blood decreases due to increased resistance.
Step-by-step explanation:
The diameter of a blood vessel has a significant impact on its resistance and flow. When the diameter of a vessel increases, there is less blood contacting the vessel wall, resulting in lower friction and lower resistance. As a result, the flow of blood through the vessel increases. On the other hand, when the diameter of a vessel decreases, more blood contacts the vessel wall, leading to increased resistance and subsequently decreasing flow.