Response to Exercise (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Effect of exercise

• Muscles must contract for the body to move, and contraction requires energy from respiration

• When exercising, the number of muscle contractions increases, so there is a greater energy demand and the muscles need a greater supply of oxygen for aerobic respiration

• The human body reacts to the increased demand for oxygen in a number of ways:

  • The breathing rate and breath volume increase during exercise to increase the amount of oxygen absorbed into the bloodstream by diffusion, and the amount of carbon dioxide removed

  • The heart rate increases

• These reactions increase the supply of oxygenated blood to the muscles

Anaerobic exercise

• If exercising vigorously, the body may not be able to supply sufficient oxygen to the muscles to meet the demand for energy

• In this situation, some of the energy supplied to the muscles comes from anaerobic respiration, which is the incomplete breakdown of glucose without oxygen

• This releases much less energy than aerobic respiration and results in the formation of lactic acid as glucose is incompletely oxidised

• An 'oxygen debt' is created after exercising in this way; it is a debt as 'extra' oxygen is needed to react with the lactic acid produced via anaerobic respiration

• This is why an individual still breathes heavily at an increased rate (with a higher heart rate than usual) after exercising vigorously

• 'Extra' carbon dioxide also has to be removed from the body

• During long periods of vigorous activity, muscles become fatigued and stop contracting efficiently as a result of increased levels of lactic acid building up

Investigations into the effect of exercise on the body

• It is relatively simple to investigate the effects of exercise on the body in the classroom

• Breathing rate can be measured by counting the number of breaths per minute, while heart rate can be measured by taking a pulse

• Either can be measured before and after an activity is performed and the results plotted on a bar chart

• It is important that the time over which breathing rate and pulse rate are measured is consistent, and that individuals fully recover (rest) before starting a new activity

Oxygen debt

Higher Tier Only

• The body can deal with lactic acid in one of two ways:

  • It can be oxidised (reacted with oxygen) to form carbon dioxide and water – the same products formed in aerobic respiration

  • Alternatively, blood flowing through the muscles transports the lactic acid to the liver where it is converted back into glucose