When outside temperature is too low, blood vessels will:

When the outside temperature is too low, blood vessels contract to help conserve body heat. This physiological response, known as vasoconstriction, reduces blood flow to the skin and extremities, which minimizes heat loss and helps maintain the body's core temperature. By narrowing the blood vessels, the body effectively redirects blood flow to vital organs, ensuring that they remain warm and functional. This mechanism is critical in preventing hypothermia and maintaining overall thermal homeostasis in cold environments.

The other options, while they describe various reactions that can occur in the body under different circumstances, do not accurately reflect the body's response to cold temperatures. For instance, expanding or dilating blood vessels typically occurs in response to heat or during physical exertion when increased blood flow is needed. Similarly, while blood vessels can also relax, this term is more ambiguous and does not specifically indicate the contraction that occurs in response to cold. Thus, contracting is the most appropriate response in this scenario.