Aerobic and anaerobic respiration
Not enough oxygen may reach the muscles during exercise. When this happens, they use anaerobic respiration to obtain energy.
Anaerobic respiration involves the incomplete breakdown of glucose. It releases around 5% of the energy released by aerobic respiration, per molecule of glucoseglucose: A simple sugar made by the body from food, which is used by cells to make energy in respiration. The waste product is lactic acidlactic acid: A toxic chemical produced during anaerobic respiration rather than carbon dioxide and water:
glucose → lactic acid (+ little energy)
Muscles become fatigued (tired) during long periods of vigorous activity. This means that they stop contracting efficiently. One cause of this is the build-up of lactic acid in the muscles from anaerobic respiration. The lactic acid is removed from the muscles by blood flowing through them.
Fit people are able to carry out physical activities more effectively than unfit people. Their pulse rate is likely to return to normal more quickly after exercise.
But being fit is not the same as being healthy. Healthy people are free from disease and infection - they may or may not be fit as well. It is possible to be fit but unhealthy, or healthy but unfit.
Much less energy is released during anaerobic respiration than during aerobic respiration. This is because the breakdown of glucose is incomplete.
Anaerobic respiration produces an oxygen debt. This is the amount of oxygen needed to oxidiseoxidation: Oxidation is a reaction in which oxygen combines with a substance. Oxidation also means a loss of electrons. lactic acid to carbon dioxide and water. The existence of an oxygen debt explains why we continue to breathe deeply and quickly for a while after exercise.
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