How does the respiratory system supply oxygen and remove carbon dioxide during exercise?
The mechanics of breathing, lung volumes and capacities, gaseous exchange at the alveoli and muscles, the control of ventilation, and the respiratory responses and adaptations to exercise and training.
A focused answer to AQA A-Level PE applied anatomy on the respiratory system, covering the mechanics of breathing, lung volumes, gaseous exchange, partial pressure gradients, neural control of ventilation and respiratory adaptations to training.
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What this dot point is asking
AQA wants you to explain the mechanics of breathing at rest and in exercise, define lung volumes and capacities, explain gaseous exchange at the alveoli and at the muscles using partial pressure gradients, describe the neural control of ventilation, and link respiratory responses and adaptations to exercise.
Mechanics of breathing
Air moves because of pressure differences created by the respiratory muscles, an application of Boyle's law, where increasing the volume of the thoracic cavity lowers the pressure inside it so air flows in, and decreasing the volume raises pressure so air flows out. At rest, inspiration is active (the diaphragm flattens and the external intercostals raise and rotate the ribs up and out, increasing thoracic volume and lowering the intrapulmonary pressure below atmospheric); expiration is passive (the muscles relax and the elastic recoil of the lungs expels air). During exercise, inspiration is more forceful and also recruits the sternocleidomastoid, scalenes and pectoralis minor to lift the ribs further, and expiration becomes active, using the internal intercostals to pull the ribs down and in and the abdominal muscles to push the diaphragm up, forcing air out faster.
Lung volumes and capacities
Gaseous exchange
The large alveolar surface area, thin walls (one cell thick) and rich capillary network maximise diffusion, in line with Fick's law, which states that the rate of diffusion increases with surface area and the partial pressure gradient and decreases with the thickness of the exchange surface. At the muscles, myoglobin has a high affinity for oxygen, storing it and aiding its diffusion to the mitochondria. The oxyhaemoglobin dissociation curve shows how readily haemoglobin gives up oxygen, and during exercise the Bohr effect (higher carbon dioxide, higher temperature and lower pH at the working muscle) shifts the curve to the right, so haemoglobin releases more oxygen where it is most needed.
Control of ventilation and adaptations
The respiratory control centre in the medulla adjusts breathing through the inspiratory and expiratory centres, responding to chemoreceptors (detecting rising carbon dioxide, falling oxygen and falling pH), baroreceptors and proprioceptors. Long-term aerobic training increases vital capacity, strengthens the respiratory muscles, lowers resting breathing frequency, increases the number of alveoli and capillaries and improves the efficiency of gaseous exchange.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksCalculate the minute ventilation of a runner whose tidal volume is 2.5 L and whose breathing frequency is 36 breaths per minute. State the units and explain what has changed compared with a resting value of about 6 L per minute.Show worked answer →
Worked calculation. Use . Substitute: L per minute. Award marks for the formula, the substitution and arithmetic, and the units (litres per minute). The explanation: minute ventilation has risen roughly fifteen-fold from rest because both tidal volume (deeper breaths) and breathing frequency (faster breaths) have increased to meet the higher oxygen demand and remove the extra carbon dioxide. Reward identifying that both components, not just one, have increased.
AQA 20224 marksExplain how oxygen is exchanged at the alveoli and unloaded at the working muscles during exercise.Show worked answer →
AO1/AO2. At the alveoli: oxygen diffuses down a partial pressure gradient from a high partial pressure of oxygen in the alveolar air to a lower partial pressure in the deoxygenated blood of the pulmonary capillaries, across the large, thin (one cell thick) respiratory surface. At the muscles: the partial pressure of oxygen in the blood is higher than in the respiring tissue, so oxygen diffuses into the muscle, where myoglobin stores it. During exercise the Bohr effect (raised carbon dioxide, temperature and acidity at the muscle) shifts the oxyhaemoglobin dissociation curve to the right, so haemoglobin releases more oxygen at the tissues. Full marks need diffusion down a partial pressure gradient named at both sites plus the Bohr effect.
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Sources & how we know this
- AQA A-level Physical Education (7582) specification — AQA (2016)