Respiration
Structures and Functions in Living Organisms
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.
Common exam question
Explaining the change in breathing rate during exercise
Question: Explain the change in breathing rate during exercise, or comment on the effect of exercise on ventilation rate using the data given (2–4 marks).
Asked in 3 of the 23 papers. Describe the data first, with numbers: the rate rises, then reaches a maximum or levels off (saying it "slows down" is ignored, and a description mark needs the time or speed at which the rise stops). Then explain: more oxygen is taken in and more carbon dioxide is released; the oxygen reaches the muscles for respiration, releasing more energy as ATP for faster muscle contraction. The depth of breathing (volume per breath) rises as well: at low effort the extra air comes from deeper breaths alone, and as the depth levels off the rate takes over. The plateau is because the chest cannot inflate and deflate any faster. Each of these is a separate mark, so give several short points rather than one long sentence.
Oxygen debt
If the exercise is so intense that even maximal breathing cannot keep up with the muscles' oxygen demand, the muscles switch to and produce . After the exercise is over, the body needs extra oxygen to break the lactic acid back down. This is the (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:
- Have each volunteer sit quietly for 1 minute, then count their number of breaths per minute at rest.
- The volunteer then performs a controlled exercise (e.g. running on the spot, or step-ups at a set pace) for 1 minute.
- Immediately after exercise, count the breaths per minute again.
- Calculate the change in breathing rate: change = breathing rate after − breathing rate before.
- 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.
Common exam question
Describing an investigation into exercise and breathing
Question: Describe a method to show the effect of exercise on breathing rate, or a simple experiment to show that exercise increases the carbon dioxide a person breathes out (3 marks).
Asked in 2 of the 23 papers. For breathing rate: count the breaths per minute at rest, then again during or straight after a set exercise; a spirometer is accepted for either count, but measuring heart rate instead loses both of those marks. Then repeat with more people and control age, sex, fitness and the amount or duration of exercise. For carbon dioxide: breathe out through limewater (or hydrogen carbonate indicator) and time how long it takes to go cloudy (or change colour) before and after exercise, keeping the volume of limewater, the room temperature and the person the same, then repeat. When a data question asks how to make the investigation more reliable, the one-mark answer is to repeat it, or use more people, and calculate a mean.