The heart, ventilation and exercise — Edexcel International A Level Biology
How the heartbeat is coordinated and monitored, how heart and ventilation rates are controlled in exercise, adrenaline, and spirometer data.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Cardiac muscle is myogenic: it contracts and relaxes rhythmically without nervous stimulation. The sinoatrial node sets the pace and sends a wave of excitation across the atria; the atrioventricular node delays it, then the bundle of His and Purkyne fibres carry it to the ventricles, which contract from the apex upwards.
An electrocardiogram (ECG) records the electrical activity of the heart from electrodes on the skin. The P wave shows atrial depolarisation, the QRS complex ventricular depolarisation and the T wave ventricular repolarisation. Changes in rate, spacing and shape help diagnose abnormal heart rhythms such as tachycardia, bradycardia, ectopic beats and atrial fibrillation.
Cardiac output is the volume of blood pumped by a ventricle per minute, calculated as stroke volume multiplied by heart rate. During exercise, chemoreceptors detect rising carbon dioxide and falling pH, and the cardiovascular control centre and ventilation centre in the medulla oblongata increase heart rate and ventilation to deliver oxygen and remove carbon dioxide rapidly.
Adrenaline is a hormone released from the adrenal glands in response to stress or danger. It binds to receptors on target cells and prepares the body for fight or flight by increasing heart rate and stroke volume, widening the airways, redirecting blood to muscles and stimulating the liver to release glucose.
A spirometer records the volume of air breathed in and out as a trace. From it you can find tidal volume, breathing rate, respiratory minute ventilation (tidal volume multiplied by breathing rate) and oxygen consumption, and compare them at rest and during exercise. All four increase with exercise.
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Quick check questions
What does myogenic mean?
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The heart muscle can contract and relax on its own, without nerve impulses.
Where is the sinoatrial node?
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In the wall of the right atrium.
What does the QRS complex on an ECG show?
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Depolarisation of the ventricles.
How do you calculate cardiac output?
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Stroke volume × heart rate.
Which part of the brain contains the ventilation centre?
Show answer
The medulla oblongata.
Frequently asked questions
How does the heart beat without being told by the brain?
Cardiac muscle is myogenic, so it generates its own excitation. The sinoatrial node in the right atrium sets the pace and sends a wave across the atria. The atrioventricular node delays it, then the bundle of His and Purkyne fibres spread it through the ventricles. Nerves only change the rate.
How do you read an ECG?
The P wave is atrial depolarisation, the QRS complex is ventricular depolarisation and the T wave is ventricular repolarisation. The time between two R peaks gives the heart rate: 60 divided by that time in seconds. Irregular spacing or missing P waves can show abnormal rhythms such as atrial fibrillation.
What controls heart rate and breathing rate during exercise?
The medulla oblongata does. Chemoreceptors detect a rise in carbon dioxide and a fall in pH in the blood. The cardiovascular control centre increases heart rate through the sympathetic nerve to the sinoatrial node, and the ventilation centre speeds up and deepens breathing, to deliver oxygen and remove carbon dioxide.
What does adrenaline do in the fight or flight response?
Adrenaline, released from the adrenal glands, prepares the body for action. It increases heart rate and stroke volume, widens the bronchioles, redirects blood to the muscles, dilates the pupils and makes the liver release glucose from glycogen, so more oxygen and glucose reach the muscles.
How do you calculate minute ventilation from a spirometer trace?
Minute ventilation is the tidal volume multiplied by the breathing rate. Read the tidal volume from the height of one breath on the trace, count the breaths in a minute, and multiply. For example, 0.5 dm³ at 14 breaths per minute gives 7.0 dm³ per minute.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026