Final answer:
An increase in blood vessel length results in higher resistance and lower blood flow due to increased surface area and friction. Changes in vessel diameter greatly impact resistance and flow, with larger diameters decreasing resistance and increasing flow.
Step-by-step explanation:
As the length of a blood vessel increases, the resistance to blood flow also increases. This is because a longer vessel provides more surface area that the blood has to come into contact with, which increases friction and thus resistance. Conversely, when a vessel is shorter, it will have lower resistance and therefore higher blood flow. The flow rate of blood decreases as length increases due to this increased resistance.
On the other hand, changes in the diameter of blood vessels have a dramatic effect on resistance. Notably, resistance is inversely proportional to the fourth power of the radius of the vessel. Therefore, a slight increase or decrease in the vessel's diameter can cause a significant decrease or increase in resistance, respectively, which in turn drastically affects blood flow.