The narrowing of blood vessels, which reduces blood flow to the skin and helps to conserve body heat.

• A. Vasodilation
• B. Vasoconstriction
• C. Hypertension
• D. Hemorrhage

Answer: (B)

When the body needs to conserve heat, the vessels to the skin narrow, which is called vasoconstriction. This happens because the smooth muscle in the walls of small arteries and arterioles tighten under sympathetic nervous system signals, reducing the diameter. With less blood reaching the skin, heat loss by radiation and convection decreases, helping to keep core temperature stable in cold conditions. If the vessels widened (vasodilation), more warm blood would flow to the skin and heat loss would increase—opposite of the goal here. The other terms describe different things: hypertension is high blood pressure, and hemorrhage is bleeding, neither of which explain this heat-conserving mechanism.