Respiration is a complex process. It runs continuously in the human body. Respiration has two phases. namely-
1. External Respiration: The process in which O2 and CO2 are exchanged between the respiratory organs and the atmosphere is called external respiration. It is a physical process. Enzyme has no role in it. No energy is produced in this process. This is done in two stages. Inhalation and exhalation.
2. Internal Respiration: The process in which complex food is broken down into simple food under the influence of enzymes, static energy is converted into kinetic energy and gaseous exchange takes place is called internal respiration. It occurs in the cytoplasm and mitochondria of cells. In this process glucose is oxidized to produce energy and CO2 is released. Its stages are glycolysis, Krebs cycle and gaseous exchange.
