Final answer:
Vasoconstriction of arterioles increases resistance and pressure while decreasing blood flow. In venules, it reduces resistance by rounding the lumen leading to an increase in venous blood flow back to the heart.
Step-by-step explanation:
When considering the effects of vasoconstriction on arterioles and venules, it's important to recognize their distinct outcomes. For arterioles, vasoconstriction causes a reduction in the vessel's radius, which increases resistance and pressure while decreasing blood flow. In contrast, vasoconstriction of venules, due to their structurally different walls, results not only in increased pressure but also an increase in venous blood flow. This occurs because the constriction rounds out the vein's lumen, reducing the surface area blood encounters, thereby reducing resistance. As atrial pressure is generally low, this efficiently aids in the return of blood to the heart, enhancing preload, and ultimately increasing the heart's contraction strength.