When there is an absence of oxygen, anaerobic respiration takes place instead of aerobic respiration. Anaerobic respiration takes place in the cytoplasm of the cell as opposed to the mitochondria and is less efficient at producing ATP. Aerobic respiration produces up to 38 ATP molecules while anaerobic produces 2 ATP molecules per molecule of glucose. Through anaerobic respiration, energy, carbon dioxide and lactic acid/alcohol are the products of glucose breakdown. The difference between 38 ATP and 2 ATP is substantial, showing the inefficiency of anaerobic respiration and how important oxygen is throughout the body.
Silverthorn, Dee Unglaub, and Bruce R. Johnson. Human Physiology: An Integrated Approach. San Francisco: Pearson/Benjamin Cummings, 2010.