Why does body heat go up during exercise?

Body heat is increased with exercise because your body is being active, your heart rate is increasing, and the result is your body will sweat which is our way of cooling ourselves down.  As your body absorbs the sweat we produce it will result in releasing heat.  Ways to keep you cool will be to drink cold water when exercising which will aid in decreasing body temperature.  Also, be sure to wear appropriate attired to prevent from overheating. 

Your body heats up when you exercise, but actually this is the body cooling itself.

Working muscle produces heat in two ways:

First, the chemical energy is about 20 to 25 per cent efficient in turning into mechanical energy. This excess energy is lost as heat.

Second, metabolic reactions, both anaerobic and aerobic, also produce excess heat that your body needs to remove. The heat in exercising muscle causes blood vessels in the skin to dilate, increasing the blood flow to the skin. The large surface area of the skin allows all the excess heat to be lost to the surrounding air.

The hypothalamus in the brain is the body's thermostat, and receptors carry it the message of excess heat. The hypothalamus, through nerve impulses, stimulates sweat glands in the skin to produce fluid, which also comes from the increased skin blood flow. As the sweat evaporates from the skin, it removes heat and cools the body.

However, evaporation of sweat does remove fluid from the body, so drinking water and/or sport drinks will maintain fluids for blood flow and sweat production. Sports drinks also replace sodium and potassium, ions that are lost in the sweat, and also provide additional glucose which fuels anaerobic and aerobic respiration.