When the body becomes warm, what happens to the blood vessels?

• A. Vasoconstriction occurs to keep heat near essential organs.
• B. Transcellular diffusion increases to move heat.
• C. Vasodilation occurs when the body is warm, causing blood vessels to dilate to release heat.
• D. The skin's permeability barrier strengthens to reduce heat loss.

Answer: (C)

Thermoregulation relies on adjusting blood flow to the skin. When the body is warm, the blood vessels in the skin dilate, increasing blood flow to the surface. This brings more heat to the skin where it can be released to the environment through radiation, convection, and especially evaporation as sweat. Vasodilation near the surface is how the body sheds excess heat.

Vasoconstriction would do the opposite, helping conserve heat when you’re cold. Transcellular diffusion isn’t the main driver of heat transfer via the bloodstream—heat is carried chiefly by blood through convection. The skin’s permeability barrier isn’t the primary mechanism for regulating heat loss either.