What you'll learn
- How the heart coordinates atrial and ventricular contraction without conscious control.
- How to link the SAN, AVN, Bundle of His and Purkyne fibres to an ECG trace.
- How exercise changes heart rate, stroke volume and cardiac output.
- How to use ECG and exercise data in calculations and explanations.
The starting point: what the heart must achieve
Your heart has four chambers: two atria at the top, which receive blood, and two ventricles at the bottom, which pump blood out of the heart.
For efficient pumping, the atria must contract first to top up the ventricles. Then the ventricles contract from the bottom upwards, forcing blood into the arteries. The relaxation phase is called diastole.
The challenge is timing. Cardiac muscle cells must be excited in the right order so the chambers contract as a coordinated pump, not as a random twitching mass.
The heart is myogenic
Cardiac muscle is unusual because it can generate its own electrical excitation.
Myogenic
Myogenic means the muscle can initiate its own contraction. In the heart, the rhythm starts in cardiac muscle tissue itself, not in a motor neurone from the brain.
The nervous system still matters, but it modifies the heart rate rather than starting every beat.
The pacemaker and conduction pathway
The sinoatrial node (SAN) is a small region of tissue in the wall of the right atrium. It is the heart’s natural pacemaker, meaning it sets the basic rhythm.
The sequence is:
- The SAN generates a wave of electrical excitation.
- The wave spreads across both atria, causing atrial systole: contraction of the atria.
- The wave reaches the atrioventricular node (AVN), which delays the impulse for about 0.1 s.
- The impulse travels down the Bundle of His, a conducting pathway in the septum between the ventricles.
- The impulse spreads through Purkyne fibres in the ventricle walls, causing ventricular contraction from the apex upwards.
The diagram links the conduction pathway to the ECG trace you will interpret in questions.
Order of excitation
The SAN makes the atria contract first; the AVN delay allows the ventricles to fill; the Bundle of His and Purkyne fibres make the ventricles contract from the bottom upwards.
Nerves do not start each heartbeat
Do not write that the brain sends an impulse to make each heartbeat happen. The heart is myogenic; sympathetic and parasympathetic nerves adjust the rate set by the SAN.
ECGs: recording electrical activity
An electrocardiogram (ECG) is a recording of the electrical activity of the heart detected at the body surface using electrodes.
Depolarisation and repolarisation
Depolarisation is a change in membrane potential that triggers cardiac muscle contraction. Repolarisation is the return of the membrane potential towards its resting state, allowing relaxation to follow.
A normal ECG has three main features:
- The P wave shows atrial depolarisation.
- The QRS complex shows ventricular depolarisation.
- The T wave shows ventricular repolarisation.
Atrial repolarisation happens during the QRS complex, but it is usually hidden because ventricular depolarisation produces a much larger signal.
ECG waves are not heart sounds
An ECG shows electrical activity, not the sound of valves closing and not the pressure changes directly. Electrical events happen just before the mechanical contraction they trigger.
Calculating heart rate from an ECG
Each R peak usually corresponds to one heartbeat, so the time between R peaks can be used to calculate heart rate.
HR=60tHR = \frac{60}{t}HR=t60where HRHRHR is heart rate in beats min⁻¹ and ttt is the R–R interval in seconds.
Calculating heart rate from an ECG
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If the R–R interval is 0.80 s, substitute into the equation:
HR=600.80=75 beats min−1HR = \frac{60}{0.80} = 75\ \text{beats min}^{-1}HR=0.8060=75 beats min−1 -
If after exercise the R–R interval falls to 0.40 s, repeat the same calculation:
HR=600.40=150 beats min−1HR = \frac{60}{0.40} = 150\ \text{beats min}^{-1}HR=0.4060=150 beats min−1 -
Compare the two values: the heart rate has doubled because the time for one cardiac cycle has halved.
Cardiac output: linking rate and volume
Heart rate is the number of heartbeats per minute. Stroke volume is the volume of blood pumped out of one ventricle per beat. Cardiac output is the volume of blood pumped out of one ventricle per minute.
Cardiac output
Cardiac output is calculated using CO=HR×SVCO = HR \times SVCO=HR×SV, where COCOCO is cardiac output, HRHRHR is heart rate and SVSVSV is stroke volume.
During exercise, cardiac output increases because heart rate increases and stroke volume usually increases too. This sends more oxygen and glucose to muscles and removes carbon dioxide faster.
Comparing cardiac output at rest and during exercise
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Calculate resting cardiac output for a heart rate of 60 beats min⁻¹ and a stroke volume of 90 cm³ beat⁻¹:
CO=60 beats min−1×90 cm3 beat−1=5400 cm3 min−1=5.4 dm3 min−1\begin{aligned} CO &= 60\ \text{beats min}^{-1} \times 90\ \text{cm}^3\ \text{beat}^{-1} \\ &= 5400\ \text{cm}^3\ \text{min}^{-1} \\ &= 5.4\ \text{dm}^3\ \text{min}^{-1} \end{aligned}CO=60 beats min−1×90 cm3 beat−1=5400 cm3 min−1=5.4 dm3 min−1 -
Calculate exercise cardiac output for a heart rate of 160 beats min⁻¹ and a stroke volume of 125 cm³ beat⁻¹:
CO=160 beats min−1×125 cm3 beat−1=20000 cm3 min−1=20.0 dm3 min−1\begin{aligned} CO &= 160\ \text{beats min}^{-1} \times 125\ \text{cm}^3\ \text{beat}^{-1} \\ &= 20000\ \text{cm}^3\ \text{min}^{-1} \\ &= 20.0\ \text{dm}^3\ \text{min}^{-1} \end{aligned}CO=160 beats min−1×125 cm3 beat−1=20000 cm3 min−1=20.0 dm3 min−1 -
Calculate the percentage increase:
20.0−5.45.4×100=270%\frac{20.0 - 5.4}{5.4} \times 100 = 270\%5.420.0−5.4×100=270%
How exercise changes cardiac control
During exercise, muscle cells respire faster. They need more ATP, so they use more oxygen and glucose. They also produce more carbon dioxide, and during intense exercise they may produce lactate and extra hydrogen ions, lowering blood pH.
The cardiovascular control centre in the medulla oblongata helps coordinate the response. It receives information from chemoreceptors, which detect chemical changes such as increased carbon dioxide concentration and decreased pH.
The response is part of the autonomic nervous system, which controls involuntary effectors:
- The sympathetic nervous system increases heart rate and contraction strength.
- The parasympathetic nervous system, mainly via the vagus nerve, decreases heart rate.
Adrenaline, released from the adrenal medulla, also increases heart rate and stroke volume during exercise.
This flowchart shows the negative feedback loop during exercise.
Responding to a fall in blood pH
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A fall in blood pH is detected by chemoreceptors in the carotid bodies, aortic bodies and medulla, increasing the frequency of nerve impulses to the cardiovascular control centre.
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The medulla increases sympathetic stimulation and reduces parasympathetic stimulation to the SAN, so the SAN depolarises more frequently; sympathetic stimulation also increases cardiac muscle contraction strength.
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Heart rate and stroke volume rise, increasing cardiac output, so carbon dioxide is removed faster and more oxygen is delivered to respiring muscles.
Recovery after exercise
After exercise stops, heart rate and ventilation do not immediately return to resting levels. This is because the body is restoring internal conditions.
Excess post-exercise oxygen consumption (EPOC) is the extra oxygen used after exercise to help restore the body to its resting state. This includes re-oxygenating haemoglobin and myoglobin, oxidising lactate, restoring ATP and phosphocreatine stores, and removing excess carbon dioxide.
A fitter person often has a lower resting heart rate and a faster recovery because their stroke volume, capillary supply and aerobic capacity tend to be higher.
Reading exercise recovery graphs
A rapid fall back towards resting heart rate usually suggests better recovery. Compare both the peak heart rate and the time taken to return near baseline, not just one point on the graph.
Practical and data-handling skills
You may be given data from a heart-rate monitor, ECG sensor or exercise test. Look for the independent variable, such as exercise intensity or time after exercise, and the dependent variable, such as heart rate, R–R interval or cardiac output.
Good investigations control factors such as age, fitness level, caffeine intake, recent exercise, temperature and exercise duration. Repeats help identify anomalous results and allow a mean to be calculated.
Always think about safety and ethics: participants should warm up, stop if they feel unwell, and give informed consent.
In the exam
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Link structure to timing: SAN → atria → AVN delay → Bundle of His → Purkyne fibres → ventricles contract from apex upwards.
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For ECG questions, state the electrical event first, then the mechanical event it causes shortly afterwards.
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In exercise explanations, connect the chain fully: increased respiration → more CO₂ or lower pH → chemoreceptors → medulla → autonomic output → increased cardiac output.
Check yourself
- Why is the AVN delay important for efficient pumping?
- How would you calculate heart rate from an R–R interval of 0.60 s?
- Why does heart rate remain raised for a while after intense exercise?
