During exercise, the muscles pull oxygen out of the bloodssstream, blood vessels dilate for increased blood flow, the heart and diaphragm work harder, and the body starts to sweat to remove heat from the body. On the molecular level, the body creates adenosine triphosphate (ATP) to provide energy to the exercising muscles.
During exercise, cells inside the muscles split the muscles' ample supply of glycogen into glucose, which then is split again to form ATP and lactic acid; this process requires about 12 different chemical reactions. ATP provides energy to the muscles' cells. The lactic acid, which is a by-product of these chemical reactions, causes muscles to ache and must be removed via the bloodstream.
Because muscles pull oxygen out of the bloodstream during exercise, the body must respond by increasing the blood flow from the heart and diverting blood flow from other organs to the working muscles. For instance, the digestive system typically slows down during strenuous exercise. As a working muscle uses up its store of ATP, it releases chemical by-products that cause blood vessels to dilate, thus increasing oxygen flow throughout the body. Blood vessels to the skin also dilate, allowing heat to dissipate. In addition, sweat glands activate during exercise to produce sweat. As the sweat evaporates upon contact with the air, it also helps to lower the body's temperature through the dissipation of built-up heat.