What substance primarily increases following the dilation of blood vessels in response to heat?

When blood vessels dilate in response to heat, a physiological process known as vasodilation occurs. This process primarily increases blood flow to the surface of the skin, which facilitates heat loss from the body. As blood flows closer to the surface, heat energy is transferred from the blood to the environment, thereby cooling the body effectively.

This increase in heat loss is critical for maintaining body temperature within a healthy range, especially during circumstances that induce heat, like exercise or hot weather. While options like oxygen supply and nutrient absorption may be indirectly affected by increased blood flow, they are not the primary responses associated with the dilation of blood vessels in relation to heat. Similarly, pH levels do not vary significantly in this context — they are more closely regulated by other physiological mechanisms. Therefore, the correct answer highlights the body's effort to regulate temperature by promoting heat loss through increased blood flow to the skin's surface.