Your body system needs energy to function and we get energy from chemical reactions. Chemical reactions are when bonds are created or existing bonds are broken between atoms. When this happens, when reacting substances are rearranged to form different substances, also called products. Each of the body cells are a chemical factory for these reactions, and in order for the cell to function, it must do work. Work is when there is a movement or change in structure, but this takes energy. So we store potential energy and chemical bonds to be transported through the body, then use kinetic energy to perform the work. No energy conversion is 100% efficient though, so heat is created. For example, when skeletal muscles contract, they perform work. The chemicals in these cells have potential energy in the chemical bonds, then they are broken and converted to kinetic energy, moving the muscle and releasing heat. Heat is related to work, so the more we work, the hotter the body gets. Metabolism is all of these reactions in the body, and we use a lot of energy daily.