Investigating the Effects of Physical Activity on Breathing (Cambridge O Level Biology)

• Exercise increases the frequency and depth of breathing
• This can be investigated by counting the breaths taken during one minute at rest and measuring average chest expansion over 5 breaths using a tape measure held around the chest
• Exercise for a set time (around 4 minutes)
• Immediately after exercising, count the breaths taken in one minute and measure the average chest expansion over 5 breaths
• Following exercise, the number of breaths per minute will have increased and the chest expansion will also have increased
• Data on breathing rate can be shown in a graph like this
  • The relative physical fitness of an individual can be compared using this method, although other data will be needed to assess fitness fully

The Link Between Breathing Rate and Exercise Graph

Variation in breathing rate during and after a period of exercise

Explaining the Link Between Physical Activity & Breathing

• Frequency and depth of breathing increase when exercising
• This is because muscles are working harder and respiring aerobically more and they need more oxygen to be delivered to them (and carbon dioxide removed) to keep up with the energy demand
• If they cannot meet the energy demand they will also respire anaerobically, producing lactic acid
• After exercise has finished, the lactic acid that has built up in muscles needs to be removed as it lowers the pH of cells and can denature enzymes that are needed for catalysing cell reactions
• It can only be removed by combining it with oxygen - this is known as ‘repaying the oxygen debt’
• This can be tested by seeing how long it takes after exercise for the breathing rate and depth to return to normal
  • The longer it takes, the more lactic acid is produced during exercise and the greater the oxygen debt that needs to be repaid

Mechanism for increasing breathing during exercise

• The rate of respiration in muscle cells increases due to exercise
• CO₂ is a product of aerobic respiration, so CO₂ is produced by the muscle cells
• CO₂ diffuses out of the muscle cells into the blood plasma
• The blood flows around the circulatory system and the increased carbon dioxide levels are detected by the brain
• The brain sends nerve impulses to the diaphragm and the intercostal muscles to increase the rate and depth of muscle contraction
• The rate of inspiration increases, along with the volume of air moved in and out with each breath
• The result is greater absorption of oxygen and removal of carbon dioxide