How does the respiratory system take in oxygen and exchange gases during exercise?
The mechanics of breathing, lung volumes and minute ventilation, gaseous exchange at the alveoli and muscles by diffusion, and the regulation of breathing during exercise.
A focused answer to OCR A-Level PE on the respiratory system: the mechanics of inspiration and expiration, tidal volume and minute ventilation, gaseous exchange at the alveoli and muscle by diffusion down partial-pressure gradients, the oxyhaemoglobin dissociation curve and the Bohr shift, and the neural and chemical control of breathing.
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What this dot point is asking
OCR wants you to describe the mechanics of breathing at rest and during exercise, calculate minute ventilation, explain gaseous exchange at the alveoli and the muscles in terms of partial-pressure gradients, interpret the oxyhaemoglobin dissociation curve and the Bohr shift, and explain how breathing is controlled.
The mechanics of breathing
Lung volumes and minute ventilation
The key volumes are tidal volume (the air moved per breath at rest, about 0.5 litres), inspiratory and expiratory reserve volumes (the extra you can breathe in or out), and residual volume (the air that always remains in the lungs). During exercise tidal volume rises by using the reserve volumes, while residual volume stays roughly the same.
At rest, litres per minute is typical. In maximal exercise tidal volume might rise to 3 litres and frequency to 50 breaths per minute, giving litres per minute.
Gaseous exchange and partial pressures
Gases move by diffusion from a high partial pressure to a low one, across the thin, moist, large-surface-area alveolar and capillary walls.
The oxyhaemoglobin dissociation curve and the Bohr shift
The oxyhaemoglobin dissociation curve is an S-shaped graph of the percentage saturation of haemoglobin against the partial pressure of oxygen. At the high partial pressures in the lungs, haemoglobin is almost fully saturated; at the low partial pressures in the muscle, it gives up its oxygen.
Control of breathing
Breathing is controlled by the respiratory centre in the medulla, split into inspiratory and expiratory areas. Chemoreceptors detect a rise in carbon dioxide and a fall in pH (the strongest stimulus), proprioceptors and mechanoreceptors detect movement in the muscles and joints, and stretch receptors in the lungs prevent over-inflation. The result is that ventilation rises sharply at the onset of exercise and falls during recovery.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20193 marksA swimmer has a tidal volume of 0.5 litres and a breathing frequency of 12 breaths per minute at rest. Calculate their minute ventilation, give the unit, and state how it changes during exercise.Show worked answer →
A Component 01 calculation. One mark for the value, one for the unit, one for the change.
Minute ventilation is , so litres per minute. During exercise minute ventilation increases (to over 100 litres per minute in trained athletes) because both tidal volume and breathing frequency rise to take in more oxygen and remove more carbon dioxide.
A common error is multiplying by the wrong frequency or omitting the unit (litres per minute).
OCR 20216 marksExplain how gaseous exchange occurs at the alveoli and at the muscles, and how the oxyhaemoglobin dissociation curve helps deliver oxygen during exercise.Show worked answer →
A Component 01 extended-response question. Markers reward partial-pressure gradients (AO1) and the Bohr shift applied to exercise (AO2).
Award marks for: at the alveoli the partial pressure of oxygen is high and that in the deoxygenated blood is low, so oxygen diffuses from the alveoli into the blood down the gradient and binds to haemoglobin; carbon dioxide diffuses the other way. At the muscles the partial pressure of oxygen in the blood is high and that in the working muscle is low, so oxygen diffuses out and is used. The S-shaped oxyhaemoglobin dissociation curve shows how saturation depends on partial pressure. During exercise the curve shifts right (the Bohr shift): rising temperature, rising carbon dioxide and falling pH make haemoglobin release oxygen more readily at the muscle, increasing delivery exactly where it is needed.
A top answer states the direction of each diffusion gradient and explains that the Bohr shift increases unloading at the tissues, not loading at the lungs.
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Sources & how we know this
- OCR A Level Physical Education (H555) specification — OCR (2016)