This is because only about 20% of the energy produced by contracting muscles is utilized for muscular contraction; the remaining 80% is converted to heat energy, and exercise raises muscle temperature. The circulation distributes heat to the body, raising body temperature. The human body is a very efficient machine for converting food into energy. The main site for this conversion is called the mitochondria, which are structures within cells that produce energy (adenosine triphosphate, or ATP). The more active your muscles are, the more fuel they need to function properly. Thus, you can see that overexerting yourself will result in your body producing more heat than normal. The more intense the activity, the higher your body temperature will rise. For example, if you run up a hill quickly, you will be using more oxygen than if you were running down it slowly, so you will be burning more calories per minute. This means that you will be generating more heat than if you were running down the hill calmly. Heat is given off as water vapor through our lungs and skin, but also remains in the blood stream, causing increased heart rate and breathing rate. At first, as your body adjusts to the change in lifestyle, you will not be able to keep up with these increased rates. Your body will then adjust by slowing its rate of metabolism so it has time to recover between workouts.