When outside temperature is too high, blood vessels will:

When the outside temperature is too high, blood vessels will dilate. This process, known as vasodilation, occurs as the body's way of regulating temperature. When the body detects elevated temperatures, it sends signals to the blood vessels to relax and widen, allowing more blood to flow near the skin's surface. This increased blood flow helps to dissipate heat from the body more effectively through the skin, which can help lower overall body temperature.

Dilation of the blood vessels is essential for thermoregulation, as it enhances the cooling effect of sweat evaporation, thus preventing overheating.