A small rise in body temperature is detected by sensory receptors in the skin and by special neurons, which monitor blood temperature, in the hypothalamus itself. When the neural centre of the hypothalamus involved in lowering body temperature receives information that body temperature is too high, it commands the arteries of the skin to dilate so that more blood, and therefore more heat, reaches the surface where heat can be lost.
A large rise in body temperature causes the hypothalamus to command both an extreme dilation in the arteries of the skin and the release of sweat from the body.