The tern is used to describe all the processes associated with the release of energy in the body. The blood carries food and oxygen to the cells so that the cells can produce energy for their needs. About a quarter of a million tons of oxygen are used by the body each year. Carbon dioxide is a byproduct of cellular metabolism. But it is not just a waste product.
Carbon dioxide is necessary for plant respiration.
Gas exchange occurs in the lungs. The right side of the heart pumps blood with a high concentration of carbon dioxide into the lungs. The carbon dioxide is replaced with oxygen. The blood changes from a dark red to a bright red color, indicating that hemoglobin has picked up the oxygen. The oxygen-enriched blood is pumped through the left side of the heart and then circulated throughout the body.