The circulatory system uses blood to move oxygen through the body. The oxygen plays a crucial role in the process of making energy for life. When blood supply diminishes, tissues respond initially by extracting more oxygen. After this strategy runs its course, cells will begin the process of anaerobic metabolism: the production of energy without oxygen. This process produces carbon dioxide and lactic acid as waste. Anaerobic metabolism cannot be sustained indefinitely, and when both sources of energy are depleted cells begin to die.