How Exercise Works

Exercise and Body Heat

Your body heats up when you exercise, and it will show on your skin. Your skin feels hotter to the touch and may look flushed, and you sweat. Although those things let you know how much heat your body is giving off, they are actually the ways that the body cools itself.

Working muscle produces heat in two ways:

The chemical energy used in muscles contracting is not efficiently turned into mechanical energy. (It is about 20 to 25 percent efficient.) The excess energy is lost as heat.
The various metabolic reactions (anaerobic, aerobic) also produce heat.

Your body needs to remove this excess heat. The heat produced by exercising muscle causes blood vessels in the skin to dilate, which increases the blood flow to the skin. This elevated blood flow to the skin and the large surface area of the skin allows the excess heat to be lost to the surrounding air.

Also, receptors carry the message of excess heat to your body's thermostat, the hypothalamus in the brain. Nerve impulses from the hypothalamus stimulate sweat glands in the skin to produce sweat. The fluid for the sweat also comes from the increased skin blood flow. The sweat evaporates from the skin, removing heat and cooling the body. Evaporation of sweat removes fluid from the body, so it is important to maintain fluids for blood flow and sweat production by drinking water and/or sport drinks. Sports drinks also replace ions (sodium, potassium) that are lost in the sweat, and provide additional glucose to fuel anaerobic and aerobic respiration.