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Respiration: 3. Onset of anaerobic respiration in animals, oxygen debt and build up of lactic acid and muscle fatigue and 'cramps'

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INDEX of biology notes on respiration

(3A) Anaerobic respiration in animals and oxygen debt

When doing vigorous exercise your body cannot supply enough oxygen to your muscles for 100% aerobic respiration.

If there is a lack of oxygen ('anaerobic' means 'without oxygen') you cannot oxidise the glucose sugar completely, as in the case of aerobic respiration - the greatly simplified equation for the incomplete breakdown of glucose is:

glucose  ===>  lactic acid  +  energy

C6H12O6   ===>  2C3H6O3  +  energy

(structure of lactic acid is CH3CH(OH)COOH, a carboxylic acid with an alcohol group)

This anaerobic reaction only partially breaks down the glucose to lactic acid in animals and some bacteria.


(i) The waste product is lactic acid, not carbon dioxide and water, as in aerobic respiration.

(ii) The products are different in plants and some microorganisms (see next section).

This is not as efficient as aerobic respiration and considerably less ATP is formed, reducing the potential energy supply.

You only make 2 molecules of ATP per molecule of glucose (much less than 1/10th of ATP from aerobic respiration).

BUT, it does enable cells to keep functioning if there is a lack of oxygen and enough energy can be released to keep a cell alive!

A much less efficient process of transferring energy from the glucose chemical energy store.

Anaerobic respiration takes place in the cytoplasm plant and animal cells and some microorganism e.g.

(a) In human cells, when you do vigorous exercise, you body can't supply enough oxygen, so the cells use anaerobic respiration too.

(b) If plant root cells are growing in waterlogged ground, there is little oxygen available, so they must respire aerobically.

(c) If bacteria cells get under your skin where there is little oxygen, they can still survive by using anaerobic respiration.

In animals, if the physical activity is intense and prolonged you get the 'cramps' pains due to the build up of lactic acid, which can be painful as anaerobic respiration kicks in due to lack of oxygen.

With anaerobic respiration you do get the build up of lactic acid in the muscles because it is biochemically more difficult to oxidise and release energy.

As anaerobic respiration produces a build up of lactic acid in the muscles, you also get an oxygen debt in the muscles which can be painful e.g. you suffer from 'cramps'.

However, anaerobic respiration has the advantage of enabling the body to keep going for a limited time, even if you are a bit short of oxygen!

This can be important in an emergency situation when you need to use your muscles more than you intended.


Know and understand anaerobic respiration results in an oxygen debt that has to be repaid in order to oxidise lactic acid to carbon dioxide and water.

Know and understand that if muscles are subjected to long periods of vigorous activity they become fatigued, ie they stop contracting efficiently.

The more vigorous the exercise, the more energy you need and you need to increase your rate of respiration.

You need to breathe at a faster rate and take in larger volumes of air for the oxygen needed to sustain this increase in rate of respiration.

Your heart rate increases to get the oxygenated blood to your muscles and simultaneously remove the carbon dioxide efficiently too.

When your exercise is really vigorous there is not enough oxygen for aerobic respiration so your body responds by using anaerobic respiration too.

However, anaerobic respiration is not as energy efficient in transferring energy as aerobic respiration and if the exercise is prolonged you become fatigued.

Know that one cause of muscle fatigue is the build-up of lactic acid in the muscles from anaerobic respiration even though the blood flowing through the muscles removes the lactic acid, oxygen is used up to oxidise lactic acid to carbon dioxide and water.

You get this extra oxygen for the aerobic respiration of the lactic acid from deeper faster breathing.

(3B) More on oxygen debt an the build-up of lactic acid

During vigorous exercise the heart, lungs and limb muscles begin struggle to keep up with what you want your body wants to do (fatigue), but you can keep your muscles going longer using anaerobic respiration, at least up to a point of total fatigue - like just about staggering over the line at the end of a marathon!

Anaerobic respiration: glucose  ===>  lactic acid  +  energy

Unfortunately, when your body starts to use anaerobic respiration, you are building up lactic acid an 'oxygen debt'.

The oxygen debt is the quantity of oxygen your body needs to react with the build up of lactic acid in the cells and remove it by oxidation to carbon dioxide and water (as happens with aerobic respiration of glucose) and replace the body's reserve of oxygen in the bloodstream and cells.

This results in muscle fatigue and stop contracting efficiently.

This means your body has to be repaid with the oxygen that the muscles didn't get for complete aerobic respiration - your lungs, heart rate and bloodstream couldn't keep up with demands of aerobic respiration.

The body has a low tolerance to lactic acid, which must be removed.

The lactic acid is taken to the liver by blood and either completely oxidised to carbon dioxide and water or converted back into glucose and then glycogen - but this takes time and needs oxygen!

This means, even when you stop doing a vigorous exercise, you have to continue to breathe quite deeply to repay this oxygen debt, transfer oxygen to the cells and oxidise the lactic acid in them to the harmless waste products of carbon dioxide and water.

Repaying the oxygen debt may take hours to complete and even days after running a marathon!

As long as your body detects higher than normal levels of carbon dioxide or lactic acid your breathing rate and pulse rate will stay higher than normal until their levels are reduced to normal, i.e. when all the excess lactic acid has been oxidised to carbon dioxide and water.

Note: Your body does have another way of reducing high levels of lactic acid and carbon dioxide.

The blood flow through your muscles transports lactic acid to the liver where it is converted back to glucose - chemically the opposite reaction to anaerobic respiration.


Regular aerobic exercise can reduce your risk from some non-communicable diseases.

See the Keeping healthy - diet and exercise 

and also Keeping healthy - non-communicable diseases - risk factors for e.g. cancers 

Keywords, phrases and learning objectives for this section on respiration

Be able to describe why the switch to anaerobic respiration (from aerobic respiration) in animals and the build up of an oxygen debt of lactic acid leading to cramps and muscle fatigue.

Know that in animal aerobic respiration, lactic acid is formed from glucose.



INDEX of biology notes on respiration


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