Final answer:
The resistance of blood flow in vessels is significantly affected by vessel radius, which can rapidly change through vasoconstriction and vasodilation, having an inverse relationship with flow. Vascular tone determines diameter and resistance, while pathological changes like plaques can greatly impact blood flow and cardiovascular health.
Step-by-step explanation:
The resistance of blood flow in vessels is mainly affected by three factors: viscosity, vessel length, and vessel radius. However, the vessel radius is the factor that can change rapidly due to vasoconstriction and vasodilation, thereby having a dramatic impact on resistance and flow. Resistance is inversely proportional to the radius of the blood vessel raised to the fourth power. A small change in radius can cause a significant change in flow, with a decreased radius increasing resistance and an increased radius decreasing resistance.
Vascular tone, which is the contractile state of the smooth muscle, is the primary determinant of blood vessel diameter, affecting resistance and flow. An increased diameter results in less blood contacting the vessel wall, leading to lower friction and thus lower resistance. Conversely, a decreased diameter means more blood contacts the vessel wall, increasing resistance.
During physiological processes like exercise, selective vasodilation occurs to direct blood flow to crucial muscles and organs. In pathological conditions, such as the presence of plaques in arteries, a slight reduction in vessel radius can significantly impede blood flow, potentially requiring an increase in blood pressure, which may be hazardous to the cardiovascular system.