In humans, the products of anaerobic respiration are adenosine triphosphate (ATP), carbon dioxide and lactic acid. ATP is the molecule that provides energy to cells.
"Anaerobic" means without oxygen, and respiration refers to the processes in a cell that convert biochemical energy, such as that found in glucose, into usable energy in the form of ATP. Waste products like carbon dioxide are also produced during this process. The fermentation process in anaerobic respiration is roughly 5 percent as effective as what cells can do when they have access to oxygen. An aerobic cycle may produce between 36 and 38 ATP molecules, while anaerobic respiration only creates 2 ATP molecules. Since muscles often run out of oxygen during extreme exertion, anaerobic respiration keeps them running. In animals, including humans, the anaerobic cycle produces lactic acid, which causes muscle cramps. In order for these cramps to stop, oxygen must find its way back into the muscle again so these cells can switch back to aerobic respiration and stop building up lactic acid. Anaerobic respiration is also common in bacteria that live in environments without oxygen; depending on the bacteria, the products of their respiration include nitrite, nitrogen gas, hydrogen sulfide, methane and acetic acid.