9.1.6B5. Transport in Humans
Explaining the Effect of Physical Activity on Heart Rate — Combined Science 0653 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 15 Unit quiz
Section 1 of 6The problem exercise creates
Muscles need more energy
- Muscles need more energy Contracting muscles use energy much faster than resting ones, so they respire at a higher rate.
- Aerobic respiration needs a supply of glucose and oxygen, so both must be delivered faster.
More waste is made
- More waste is made Faster respiration produces more carbon dioxide and heat that must be carried away from the muscle cells.
Section 2 of 6How the body responds
1Muscles respire faster
- Muscles respire faster The demand for oxygen and glucose rises sharply.
2Heart rate increases
- Heart rate increases The heart beats more often (and more forcefully), so more blood is pumped each minute.
3Faster blood flow
- Faster blood flow More oxygen and glucose reach the working muscles, and breathing rate also rises to take in more oxygen.
4Wastes removed quicker
- Wastes removed quicker Carbon dioxide is carried away faster to the lungs, and heat is carried to the skin.
Section 3 of 6Why the heart stays fast after exercise
1
During hard exercise the oxygen supply may not keep up, so muscle cells also respire anaerobically to release extra energy.
2
Anaerobic respiration produces lactic acid, which builds up in the muscles and is harmful in large amounts.
3
This leaves an oxygen debt, which is the extra oxygen needed afterwards to break down the lactic acid.
4
The heart and breathing rate stay high after the activity stops so that this extra oxygen is delivered and the lactic acid is removed in the blood.
5
Once the debt has been repaid and wastes cleared, the heart rate gradually falls back to the resting value.
Section 4 of 6Linking cause and effect
| Observation | Explanation |
|---|---|
| Heart rate rises as exercise begins | Muscles need more oxygen and glucose for increased respiration. |
| Harder exercise gives a higher rate | Greater energy demand means a greater need for oxygen and for waste removal. |
| Rate stays high after stopping | Oxygen debt must be repaid and lactic acid broken down. |
| Fitter people recover sooner | Their hearts pump more blood with each beat and their muscles use oxygen efficiently, so the debt is smaller. |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Aerobic respiration | Release of energy from glucose using oxygen, producing carbon dioxide and water. |
| Anaerobic respiration | Release of energy from glucose without oxygen; in muscles it forms lactic acid. |
| Lactic acid | A waste product of anaerobic respiration in muscle cells. |
| Oxygen debt | The extra oxygen required after exercise to break down the lactic acid that has built up. |
| Recovery | The period after exercise when heart and breathing rate return to resting levels. |
Section 6 of 6Exam technique
1
Saying the heart beats faster 'to give energy'. It beats faster to deliver oxygen and glucose for respiration and to remove wastes.
2
Stating that oxygen debt is a shortage of oxygen that remains afterwards. It is the extra oxygen needed to remove lactic acid.
3
Forgetting waste removal as a reason. Carbon dioxide and lactic acid have to be carried away too.
4
Claiming the heart rate drops instantly when exercise ends. It falls gradually.
5
Mixing up aerobic and anaerobic respiration and saying lactic acid is made with oxygen present.
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