Explaining the Link Between Physical Activity & Breathing: Extended (CIE IGCSE Biology)

• Frequency and depth of breathing increase when exercising
• This is because muscles are working harder and aerobically respiring 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 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 produced during exercise and the greater the oxygen debt that needs to be repaid

Mechanism for increasing breathing during exercise

• The rate of respiration increases in muscle cells when exercising heavily
• CO₂ is a product of aerobic respiration, so CO₂ levels increase in the muscle cells
• This CO₂ diffuses out of the cells into the blood plasma
  • CO₂ in solution causes a slight drop in pH so the blood becomes slightly more acidic
• The blood flows around the circulatory system and passes to the brain where the increased carbon dioxide levels are detected by chemoreceptors in the brain
  • Chemoreceptors are cells that detect chemical changes in the body
  • They can detect changes in blood gas levels, as well as changes in pH
  • The chemoreceptors are located in the medulla oblongata of 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 the volume of air moved in and out with each breath
• The result is greater absorption of oxygen and removal rate of carbon dioxide
  • This supports the increased rate of respiration in the exercising muscle cells