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. The carbon dioxide is then exhaled.