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 oxygen absorbed into the bloodstream by diffusion

    • increase the rate of carbon dioxide removal

  • The heart rate increases

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

Anaerobic respiration during exercise

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

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

• This releases much less energy than aerobic respiration and results in the formation of lactic acid

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

• An oxygen debt is created

Investigating the effect of exercise on the body

• It is possible to investigate the effects of exercise on the body by taking measurements before and after a period of exercise

  • Breathing rate can be measured by counting the number of breaths per minute

  • Heart rate can be measured by taking a pulse

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

• Use groups of people (at least five in each group – not one person repeated), control variables such as age, sex, fitness and the type and length of exercise, calculate the increase for each person and compare the mean increases

Oxygen debt

Higher Tier Only

• Oxygen debt is:

the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells

• After exercise, the body processes this lactic acid as follows:

  • Blood flowing through the muscles transports the accumulated lactic acid to the liver

  • In the liver, the lactic acid is converted back into glucose

  • The extra oxygen taken in after exercise reacts with the lactic acid and breaks it down

• After exercise, breathing rate and heart rate stay high to supply the extra oxygen needed to break down (oxidise) the lactic acid – this repays the oxygen debt