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7.4.1Topic 7 · Run for your Life

Variations in Breathing Rate and Heart Rate — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 21 of 25 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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7.4.1 Variations in Breathing Rate and Heart Rate — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how and why breathing rate, tidal volume and heart rate change during exercise, and how these are controlled.Breathing (ventilation) moves air in and out of the lungs.

  • Tidal volume (TV)
  • Breathing (ventilation) rate
  • Minute ventilation
  • Vital capacity
  • Residual volume
  • Inspiratory and expiratory reserve volumes

Section 1 of 9Lung volumes and ventilation

Breathing (ventilation) moves air in and out of the lungs. Several measurements describe it:

1Tidal volume (TV)
Tidal volume (TV) volume of air breathed in or out in one normal breath (about 500 cm3 at rest).
2Breathing (ventilation) rate
Breathing (ventilation) rate number of breaths per minute (about 12 to 15 at rest).
3Minute ventilation
Minute ventilation tidal volume × breathing rate (about 6 to 7.5 dm3 min-1 at rest).
4Vital capacity
Vital capacity the maximum volume of air that can be exhaled after a maximum inhalation (about 4.5 to 5 dm3).
5Residual volume
Residual volume air that remains in the lungs after the strongest exhalation, so the alveoli never completely collapse.
6Inspiratory and expiratory reserve volumes
Inspiratory and expiratory reserve volumes extra air that can be inhaled or exhaled beyond a normal breath.

These volumes are measured with a spirometer. In the trace, tidal volume is the amplitude of the regular wave; breathing rate is the number of peaks per minute; and oxygen uptake can be found from the slope of the trace if the carbon dioxide is absorbed by soda lime.

Section 2 of 9Control of breathing

Ventilation is controlled automatically by the ventilation centre in the medulla oblongata.

Control of breathing
At rest it sends impulses along motor neurones to the diaphragm and external intercostal muscles, which contract, causing inhalation.
During forced breathing the centre also stimulates internal intercostals to contract and force air out.
Stretch receptors in the lungs prevent over-inflation by sending impulses that inhibit inspiration (the Hering-Breuer reflex).

The ventilation centre responds to the concentration of carbon dioxide in the blood.

Chemoreceptors
Dissolved CO2 forms carbonic acid, which lowers the pH. Chemoreceptors in the medulla, the carotid bodies and the aortic bodies detect the fall in pH and send impulses to the medulla.
The medulla then increases the frequency and strength of impulses to the diaphragm and intercostal muscles, so both breathing rate and depth (tidal volume) increase.
This removes CO2 more rapidly and raises blood pH back to normal.
It is a negative feedback loop.

Section 3 of 9What happens during exercise

Working muscles respire more quickly, so they use more oxygen and produce more CO2 (and lactate when respiration is partly anaerobic). The blood pH falls, and the chain of events is:

1
Chemoreceptors detect increased CO2 / decreased pH.
2
More impulses go to the cardiovascular and ventilation centres in the medulla.
3
The ventilation centre increases the rate and depth of breathing, so more oxygen enters the blood and more CO2 leaves.
4
The cardiovascular centre increases sympathetic stimulation of the SAN, so heart rate and stroke volume increase, raising cardiac output and delivering oxygen and glucose to the muscles faster.

Heart rate also rises before exercise starts because of adrenaline released in anticipation, and the rate falls slowly during recovery as the oxygen debt is repaid. Proprioceptors in muscles and joints also detect movement and stimulate the medulla, so breathing increases at the very start of exercise, before CO2 concentration changes.

Section 4 of 9Comparing rest and exercise

MeasurementRestHeavy exercise
Heart rate (beats per min)about 70up to about 190
Breathing rate (per min)about 12about 40 or more
Tidal volume (cm3)about 500about 3000
Minute ventilation (dm3 min-1)about 6over 100

Increased ventilation maintains a steep concentration gradient of oxygen and carbon dioxide across the alveolar membrane, so gas exchange is faster. The increases are proportional to how hard the body works, up to a limit.

Section 5 of 9Interpreting data

If given a graph of breathing rate and blood CO2 concentration against time, note that breathing rate rises after CO2 rises and falls after the exercise stops, with a delay of some seconds.

Interpreting data
Quote values with units, calculate the percentage change and explain every trend with the control mechanism.
If breath is held voluntarily, CO2 builds up and, once chemoreceptors detect it, the urge to breathe overrides conscious control.
This shows that carbon dioxide, not low oxygen, is the main stimulus for breathing in healthy people.

Section 6 of 9Exam approach

Answers on exercise physiology score well when they follow the chain stimulus, receptor, coordinator, effector, response.

Exam approach
Name the receptors (chemoreceptors for CO2 and pH, baroreceptors for pressure, proprioceptors for movement), the coordinator (medulla oblongata), the nerves (sympathetic and vagus to the heart; phrenic and intercostal nerves to the breathing muscles) and the effectors (SAN, diaphragm, intercostal muscles).
Always state the direction of change and the reason it helps: faster ventilation maintains the concentration gradient for gas exchange, and faster circulation transports oxygen to respiring muscle.

Section 7 of 9Exam-style questions

  1. A person's tidal volume is 520 cm3 and breathing rate is 14 breaths per minute. Calculate the minute ventilation in dm3 min-1. [2]2 marks

  2. Explain how breathing rate increases during exercise. [4]4 marks

  3. Explain why heart rate and breathing rate both increase during exercise. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Tidal volumeVolume of air in one normal breath
Minute ventilationTidal volume multiplied by breathing rate
Vital capacityMaximum volume of air that can be exhaled after a maximum inhalation
Residual volumeAir left in the lungs after maximum exhalation
SpirometerApparatus that records breathing volumes
Ventilation centreRegion of the medulla that controls breathing
ChemoreceptorReceptor that detects change in blood CO2 or pH
ProprioceptorReceptor that detects movement of muscles and joints

Section 9 of 9Exam technique

1
Saying low oxygen is the main stimulus for faster breathing: it is a high CO2 (low pH).
2
Confusing tidal volume with vital capacity.
3
Saying that the lungs are completely emptied at the end of exhalation: residual volume remains.
4
Forgetting that both rate and depth of breathing increase.
5
Saying that the heart rate rise is only due to adrenaline.

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