Respiration - The Introduction Respiration, chemical and physical processes by which animals and plants obtain oxygen and use it to release energy from food molecules. Respiration provides the necessary energy for carrying on all essential life processes. Respiration is divided into two phases: external respiration and internal, or cellular, respiration. External respiration includes all steps in the process of delivering oxygen to each cell in the body and disposing of carbon dioxide, a gas given off as a waste product when cells use oxygen. In humans and other animals that have lungs, breathing is an essential part of external respiration.