In small arteries and arterioles, blood flow to organs is controlled by altering resistance such that:

• A. a decrease in resistance has no effect on blood flow
• B. an increase in resistance increases blood flow to the organ
• C. blood flow remains constant regardless of resistance changes
• D. an increase in resistance decreases blood flow to the organ

Answer: (D)

Verified step by step guidance

1. Understand the relationship between blood flow, resistance, and pressure. Blood flow (Q) is determined by the equation: $Q=\frac{\mathrm{\Delta P}}{R}$, where ΔP is the pressure difference and R is the resistance.
2. Recognize that resistance in blood vessels is primarily influenced by the diameter of the vessel. According to Poiseuille's law, resistance (R) is inversely proportional to the fourth power of the radius of the vessel: $R=\frac{8}{\eta }/r^4$, where η is the blood viscosity, l is the vessel length, and r is the radius.
3. Analyze the effect of increasing resistance. If resistance (R) increases while the pressure difference (ΔP) remains constant, the blood flow (Q) will decrease because of the inverse relationship between Q and R in the equation $Q=\frac{\mathrm{\Delta P}}{R}$.
4. Apply this understanding to the context of small arteries and arterioles. These vessels regulate blood flow to organs by changing their diameter (vasoconstriction or vasodilation), which alters resistance. An increase in resistance (due to vasoconstriction) reduces blood flow to the organ.
5. Conclude that the correct answer is: 'an increase in resistance decreases blood flow to the organ,' as this aligns with the physiological principles of blood flow regulation.