Which mechanism primarily reduces peripheral resistance when tissue demand increases?

• A. Dilation of arterioles, reducing vascular resistance
• B. Constriction of venules
• C. Increasing blood viscosity
• D. Decrease in heart rate

Answer: (A)

Dilation of arterioles is the key mechanism that lowers peripheral resistance when tissue demand rises. When tissues become more active, local metabolic signals cause the arteriolar smooth muscle to relax, widening the arterioles and dramatically reducing resistance (resistance falls as radius increases, due to the r^4 relationship). This vasodilation increases blood flow precisely where it’s needed to meet the higher metabolic demands. Constricting venules, increasing blood viscosity, or decreasing heart rate don’t produce this targeted drop in arteriolar resistance; they either don’t affect the resistance of the arterial tree in the same way or actually work against increasing tissue perfusion.