Lesson 6 of 6 / Respiration in humans
Anaerobic respiration and recovery
Why can rapid, deep breathing continue after a sprint ends?
I can explain anaerobic energy release, lactic acid and repayment of an oxygen debt.
I can define human anaerobic respiration and give its word equation.Syllabus
Syllabus K325, Topic 6(f). Define human anaerobic respiration and give its word equation.
I can explain exercise, oxygen debt and recovery breathing.Syllabus
Syllabus K325, Topic 6(g). Explain exercise, oxygen debt and recovery breathing.
About 5 min
Make a guess. You are not marked.
You stop sprinting. Why do you keep breathing hard for several minutes?
Show the answer
To take in extra oxygen for recovery.
Anaerobic respiration made lactic acid. Extra oxygen is needed to break it down or turn it back into glucose.
The key idea
When oxygen delivery cannot meet intense demand, anaerobic respiration contributes energy; recovery requires extra oxygen.
The curve illustrates a trend, not measured results. Breathing rate alone does not measure oxygen consumption, breath depth or lactic acid concentration.
Explanation
Anaerobic respiration releases energy by breaking down glucose without using oxygen. In human muscle cells, the word equation is glucose -> lactic acid, with a small amount of energy released compared with aerobic respiration.
During vigorous exercise, muscle demand for energy rises quickly. Oxygen delivery may not support enough aerobic respiration to meet that demand. Anaerobic respiration then contributes additional energy even while aerobic respiration continues.
Lactic acid accumulates when it is produced faster than it can be removed or used. After exercise, extra oxygen is needed for recovery. This additional requirement is described in the syllabus as an oxygen debt.
Breathing remains rapid and deep, increasing oxygen uptake. Some lactic acid is oxidised to carbon dioxide and water; some can be converted back into glucose, particularly in the liver, using energy supported by aerobic respiration. Do not describe lactic acid as a gas that is breathed out.
Heart rate also stays high after exercise. The faster heartbeat carries lactic acid in the blood from the muscles to the liver, where it is converted back to glucose. It also delivers the extra oxygen that the liver and muscles need to remove the oxygen debt. Heart rate and breathing rate fall back towards resting values as the lactic acid is removed.
The recovery response also restores the body towards its resting state. As extra demand declines, breathing moves back towards its resting rate and depth. Cells still carry out aerobic respiration at rest.
Step by step
- 1
Compare demand and delivery
Explain that oxygen delivery may be insufficient for all the energy demanded during vigorous exercise.
- 2
Add the anaerobic contribution
Name glucose breakdown, lactic acid formation and relatively little energy released per glucose.
- 3
Explain the recovery breathing
Extra oxygen supports recovery and lactic acid processing, so breathing does not return to rest instantly.
Worked example
Read a recovery trend
An illustrative breathing rate is 14 breaths/min at rest, 32 immediately after exercise, 22 after two minutes and 14 after five minutes. Explain the trend.
One way to explain it
Breathing is initially elevated to supply extra oxygen for recovery after exercise, including processing lactic acid and removing the oxygen debt. As recovery proceeds, the extra requirement declines and breathing returns towards the resting rate. Cellular respiration continues even after breathing returns to its resting rate.
Why this answer works
- Describe the measured fall from 32 to 14 breaths/min.
- Explain why the starting value remains above rest after exercise stops.
- Breathing rate alone does not measure the amount of lactic acid.
Watch out for this
A student says: "During a sprint, every cell switches off aerobic respiration completely." What is wrong with this?
Show the answer
Aerobic respiration can continue. Anaerobic respiration supplies an additional contribution when oxygen delivery cannot meet the full demand.
Check your understanding
Why does a runner keep breathing rapidly and deeply after vigorous exercise?
Show the answer
To supply extra oxygen needed for recovery and removal of the oxygen debt
The additional oxygen supports recovery after anaerobic activity, so ventilation remains elevated for a time.
Exam-style questions
Check your understanding
Invented data: a runner's breathing rate is 14 breaths/min at rest, 40 at the end of a sprint and 20 after three minutes. Why is it still above 14 after three minutes?
Show the answer
Extra oxygen is still needed to break down lactic acid
The oxygen debt is repaid after exercise by oxidising lactic acid, so breathing stays fast.
Quick recall
Why can rapid, deep breathing continue after a sprint ends?