How does the human body lose calories?

The body does not "lose" calories, per say. Instead, it uses them. Calories are what provides the body with energy. This energy is what allows things like our heart to beat, for our muscles to move, and to give us the energy to do things like walk and play sports. By doing activities, the body requires energy to do so and it uses the calories stored in our body (first it uses those stored in the liver, which are from what we recently ate / drank, then it moves to energy stored in fat cells around the body).