What is vasodilation?

• A. Decreases the diameter of blood vessels
• B. Stimulates sweat production
• C. Causes hair to stand on end
• D. Increases the diameter of blood vessels, aiding thermoregulation

Answer: (D)

Vasodilation is the widening of blood vessels, caused by relaxation of the smooth muscle in the vessel walls. This increases the diameter of the vessels, especially those near the skin, which allows more blood to flow to the surface. With more blood at the skin, heat can be released more easily, helping regulate body temperature.

The statement that describes increasing the diameter of blood vessels, aiding thermoregulation, is the best match because it directly defines what vasodilation does and its role in cooling the body.

Decreasing the diameter would be vasoconstriction, not vasodilation. Sweating helps cool the body through evaporation, not by changing vessel diameter. Hair standing on end is a separate autonomic response (piloerection) and doesn’t describe changes in blood vessel size.