Respiration

During exercise the muscles use far more ATP than at rest, so they need far more glucose and oxygen and produce far more CO₂. The body responds in several ways:

• Breathing rate increases, with more breaths per minute drawing in fresh oxygen faster.
• Depth of breathing increases, so each breath moves a larger volume of air (the "tidal volume" rises).
• Heart rate increases to pump the oxygen-rich blood to the muscles faster.

The combined effect is that more oxygen reaches the muscles per minute and more CO₂ is cleared per minute.

Oxygen debt

If the exercise is so intense that even maximal breathing cannot keep up with the muscles' oxygen demand, the muscles switch to anaerobic respiration and produce lactic acid. After the exercise is over, the body needs extra oxygen to break the lactic acid back down. This is the oxygen debt (sometimes called excess post-exercise oxygen consumption, EPOC).

You "pay back" the oxygen debt by continuing to breathe hard for several minutes after stopping, even though the muscles are no longer working. The extra oxygen is being used by your liver to convert lactic acid back into harmless products (glucose, CO₂ and water).

Investigating the effect of exercise on breathing rate

A simple practical to demonstrate this:

1. Have each volunteer sit quietly for 1 minute, then count their number of breaths per minute at rest.
2. The volunteer then performs a controlled exercise (e.g. running on the spot, or step-ups at a set pace) for 1 minute.
3. Immediately after exercise, count the breaths per minute again.
4. Calculate the change in breathing rate: change = breathing rate after − breathing rate before.
5. Repeat with several volunteers and take a mean to reduce variation.

Expected result: breathing rate is markedly higher after exercise. The increase ranges from a few breaths per minute (for light exercise) to more than double the resting rate (for vigorous exercise).

Limitations:

• Different people have different fitness levels. Control this by using a similar age and fitness range, or report results for each person separately.
• Exercise intensity is hard to standardise. Use a fixed exercise (step-ups at a metronome pace) rather than free running.
• Counting breaths immediately after exercise is fiddly. Count for 15 seconds then multiply by 4 to catch the result before the rate falls back.