The basics of metabolism are the old cells and organisms leaving the body through breathing, nutrition and secretion and replacing them with newer cells. This enables all the body functions, such as breathing, blood flow, digestion, growth and reproduction. Therefore metabolism is the most important task our bodies perform.

The decomposition of the organic substances is called catabolism, and the opposite, the new substances forming, is anabolism. In anabolism the energy we consume is formed into more complex compounds that we can use to build our bodies. In catabolism the energy is disassembled for heat production, cellular function, and for muscular movement.

Metabolism is regulated through the nervous system with neurotransmitters (neural regulation) and through the bloodstream and interstitial fluids (humoral regulation). In charge of the transportation of compounds are the respiratory and circulatory systems together with the lymphatic system.