Explaining the Link Between Physical Activity & Breathing

Why Breathing Rate and Depth Increase During Exercise

• During exercise, muscle cells work harder and undergo more aerobic respiration to meet the body’s energy demands.

• To support this, the frequency (rate) and depth of breathing increase.

• Muscles need:

  • More oxygen delivered to them.

  • More carbon dioxide removed as a waste product of respiration.

What Happens When Demand Exceeds Supply?

• If oxygen delivery cannot keep up, muscles begin to carry out anaerobic respiration.

• This leads to the production of lactic acid.

• Lactic acid lowers the pH of cells and can denature enzymes, disrupting normal cellular activity.

Oxygen Debt & Recovery

• After exercise, lactic acid must be broken down.

• This process requires oxygen, known as repaying the oxygen debt.

• The longer it takes for breathing rate and depth to return to normal, the greater the oxygen debt — indicating more lactic acid was produced during exercise.

Mechanism Behind the Increase in Breathing During Exercise

1. Increased respiration in muscles produces more carbon dioxide (CO₂).

2. CO₂ diffuses from muscle cells into the blood plasma.

3. CO₂ in the blood forms a weak acid, lowering blood pH slightly.

4. This acidic blood reaches the brain, where chemoreceptors in the medulla oblongata detect:

   • Increased CO₂ levels

   • Decreased pH

5. In response, the brain sends nerve impulses to:

   • The diaphragm

   • The intercostal muscles

6. This causes:

   • Increased rate of breathing (inspiration)

   • Greater depth of breathing

Overall Result

• More oxygen is absorbed into the bloodstream.

• More carbon dioxide is removed.

• This supports the high rate of respiration needed by active muscles during physical activity.