Anaerobic Respiration in Humans: Lactic Acid and Oxygen Debt
Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
When muscles work vigorously and oxygen supply cannot meet demand, anaerobic respiration occurs: glucose → lactic acid. It releases much less energy per glucose molecule than aerobic respiration. The lactic acid build-up causes muscle fatigue. After exercise, extra oxygen is needed to break down lactic acid: this is the oxygen debt.
Anaerobic respiration in humans is tested alongside exercise physiology. Key points: it happens in addition to aerobic respiration (not instead of it), it produces lactic acid (not alcohol), and it releases far less energy because glucose is only partially broken down.
The equation and conditions
Word equation: glucose → lactic acid (+ small amount of energy released)
Anaerobic respiration occurs in muscles during intense exercise when the blood cannot deliver oxygen fast enough for aerobic respiration alone to meet energy demands. It does not replace aerobic respiration: both happen simultaneously, but the anaerobic pathway supplements energy supply.
Unlike aerobic respiration, anaerobic respiration does not require oxygen and does not produce carbon dioxide or water. The glucose molecule is only partially broken down, so much less energy is released per molecule of glucose.
Lactic acid and muscle fatigue
The accumulation of lactic acid in muscles causes muscle fatigue: the muscles become less efficient at contracting and may feel painful or heavy. This is what limits how long a sprinter can maintain maximum effort.
Lactic acid is transported by the blood from the muscles to the liver, where it is broken down. This breakdown requires oxygen, which is why you continue to breathe heavily after intense exercise: your body is repaying the oxygen debt.
Oxygen debt explained
The oxygen debt is the extra oxygen needed after exercise to break down the lactic acid that accumulated during anaerobic respiration. The lactic acid is converted back into glucose in the liver, or it may be fully oxidised to carbon dioxide and water.
Signs of oxygen debt repayment: continued heavy breathing and elevated heart rate after exercise stops. These persist until the lactic acid has been fully broken down and oxygen levels in the muscles are restored. A fit person repays their oxygen debt faster because their cardiovascular system delivers oxygen more efficiently.
Try a focused question (4 marks)
During a 100 m sprint, a runner's muscles carry out anaerobic respiration. Explain why and describe what happens to the lactic acid produced.
Show the answer and marking guidance
The muscles need energy faster than aerobic respiration alone can supply, because oxygen delivery cannot keep up with demand.
Anaerobic respiration occurs: glucose is converted to lactic acid, releasing a small amount of energy.
After the sprint, lactic acid is transported by the blood to the liver.
In the liver, lactic acid is broken down using oxygen (the oxygen debt), which is why breathing remains heavy after exercise.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying anaerobic respiration produces alcohol in humans: alcohol is produced by yeast, not human muscles.
Claiming anaerobic respiration releases more energy than aerobic: it releases much less.
Saying lactic acid is broken down in the muscles: it is transported to the liver.
Describing oxygen debt as the oxygen used during exercise rather than the extra oxygen needed after exercise.