Oxygen is not present with anaerobic exercise. When we exercise anaerobically glycogen is used as fuel. Once all the glycogen has been depleted (usually in about two hours) you can expect to hit the proverbial wall. Endurance athletes avoid this performance buster with carbo loading before exercise (which when converted to sugar gives more energy) and supplements during exercise to sustain energy.
During anaerobic exercise your body builds up lactic acid, which causes discomfort and fatigue at sustained levels. For this reason anaerobic exercise or high intensity exercise happens in short bursts. It may be helpful to consider the difference between a sprinter (anaerobic) and a marathoner (aerobic). Sprinting is an all-out effort that is sustained for a comparatively short period while marathoning is a sustained effort.
Benefits of Anaerobic Exercise
Anaerobic exercise helps build lean muscle mass. Calories are burned more efficiently in bodies that have more muscle. Anaerobic exercise is especially helpful for weight management in that it helps to burn more calories even in a body at rest. Anaerobic exercise can also help build endurance and fitness levels.