Most organisms, including humans, require oxygen to function normally and survive. There are a few organisms that do not require oxygen and actually find it to be toxic. There is also a group of organisms that can function both in the presence and absence of oxygen. A system that requires oxygen is called an aerobic system and one that doesn’t require oxygen is called an anaerobic system.
Most of the activities of the cells of the human body need oxygen to be able to carry out their day-to-day functions. There are some tissues that are able to work briefly without oxygen though. One example is muscle cells. Muscles can work for a limited amount of time using an anaerobic system before oxygen is required. Even for a brief period of time of working anaerobically, there are quite serious repercussions that must be corrected.
For long periods of continuous activity, such as distance running, muscles will use an aerobic system and burn oxygen to provide the required energy to maintain the activity. For short bursts of vigorous activity, such as sprinting, the muscles can break down glucose molecules in the absence of oxygen. An anaerobic system is often used for a short period of time when the blood cannot carry oxygen quickly enough to the working muscle cells. Unfortunately, this system cannot be used indefinitely for a number of reasons.
Energy is stored in cells in a chemical called adenosine triphosphate, or ATP. Usually, enough ATP is stored within muscle cells to allow for about 20 – 30 seconds of activity before more has to be produced. ATP is produced during respiration by the breakdown of glucose molecules within the cells. Depending on whether oxygen is available, aerobic or anaerobic respiration will produce the ATP. The aerobic system is much more efficient and produces far more ATP, but the anaerobic system is what is used for the first minutes of exercise.
Another product of respiration is carbon dioxide. As the amount of carbon dioxide increases in the bloodstream, it stimulates the heart to pump faster. This in turn causes more oxygenated blood to be carried around the body, including to the muscle cells that require it to continue respiration and producing further ATP. If oxygen levels drop in the cells, the anaerobic system for respiration will begin again.
Lactic acid is a toxic product of the anaerobic respiration in humans. If the anaerobic system continues for any length of time, it will begin to build up in the tissues. The buildup of lactic acid causes muscles to become fatigued and can cause painful side effects like a muscle cramp. This oxygen debt must be repaid as soon as possible, once the activity has stopped. Rapid and deep breathing helps get as much oxygen into the body as possible, which is used by the cells to break the lactic acid down into carbon dioxide and water.