If you've ever exercised so hard your muscles started burning, you know about lactic acid. Lactic acid is a product of normal cellular anaerobic respiration. It used to be thought that lactic acid was simply a waste product of anaerobic metabolism. However, lactic acid can be used — and useful — even during aerobic respiration.
Lactic Acid Production
The primary means by which your body metabolizes nutrients to produce energy is aerobic respiration. Aerobic respiration utilizes oxygen to facilitate energy production. When you exercise, your breathing rate increases to keep up with the demand for extra oxygen.
However, if your breathing rate and blood flow cannot supply enough oxygen to your working muscles, your body turns to anaerobic respiration. This is the production of energy without the use of oxygen. This system works by producing lactic acid to facilitate energy production.
Anaerobic respiration can sustain energy for one to three minutes by producing lactic acid. When the body turns back to aerobic respiration, there is generally an accumulation of lactic acid.
Lactic Acid and Aerobic Respiration
Your body, being the finely tuned machine that it is, has a way of dealing with this abundance of lactate. The mitochondria of your skeletal muscle cells, the place where aerobic energy production occurs, are able to take in extra lactic acid, metabolize it and use it for energy production.
So, while aerobic respiration does not produce lactic acid for use, it is still able to use it if it has been produced through other means.
Consequences of Lactic Acid Buildup
Lactic acid, is just that, an acid. While the body can deal with some lactic acid, the accumulation of it can cause fatigue and limit your exercise time. If too much lactic acid is released into your bloodstream, it decreases the pH of your blood. In your muscle, an accumulation of lactic acid can inhibit glycolysis, the breakdown of glycogen into glucose, which is the primary fuel for energy.
Adaptations to Exercise
Over time your body can adapt to most anything you make it do. With training, the amount of lactic acid produced by your body declines. When it is produced, your body is better able to handle the lactic acid and can utilize it so that it does not accumulate in your muscles or bloodstream. In addition, your body becomes better equipped to fatigue at a higher rate of lactic acid accumulation.