5.3Unit 5: Respiration, Internal Environment, Coordination and Gene Technology

Exercise — IAL Edexcel Modular Revision Notes & Topical Questions

10 min read 12 parts Lesson 4 of 7 1 quiz

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5.3 Exercise — IAL EDEXCEL MODULAR notes: Explain how the heartbeat is initiated and controlled, how breathing and heart rate respond to exercise, how to interpret ECGs and spirometer traces, and the health and ethical aspects of exercise.Myogenic — cardiac muscle contracts witho…

Electrocardiogram (ECG)
Heart rate from an ECG
Cardiac output
It rearranges to heart rate
Resting value
Ventilation centres

Part 1 of 10Control of the heartbeat

Myogenic
- cardiac muscle contracts without an external stimulus, giving an intrinsic rhythm of about 60 beats per minute.

The sinoatrial node (SAN) in the wall of the right atrium starts a wave of depolarisation that makes the atria contract.

Non-conducting tissue stops the wave spreading straight to the ventricles.

The wave reaches the atrioventricular node (AVN), which delays the impulse so the ventricles contract after the atria.

The AVN passes the impulse down the bundle of His in the septum, which divides into two branches.

The branches carry the impulse to the Purkyne fibres, which spread round the ventricles.

The ventricles depolarise from the apex upwards, forcing blood into the pulmonary artery and aorta.

Part 2 of 10ECGs

Electrocardiogram (ECG)
- electrodes on the skin record the electrical activity of the heart.

Feature	Cause
P wave	Depolarisation of the atria, causing atrial systole
QRS complex	Depolarisation of the ventricles, causing ventricular systole; the largest wave because the ventricles have the most muscle
T wave	Repolarisation of the ventricles, causing diastole
U wave	Cause uncertain, possibly repolarisation of Purkyne fibres

Heart rate from an ECG

60 ÷ time between R peaks in seconds.

Abnormalities

Tachycardia is a resting rate above 100 beats per minute.
Bradycardia is a resting rate below 60 beats per minute, common and harmless in fit athletes.
An ectopic heartbeat is an early beat followed by a pause, common and usually needing no treatment.
Fibrillation is irregular, uncoordinated contraction of the atria or ventricles and can be fatal.

Part 3 of 10Cardiac output

Cardiac output

Cardiac output heart rate × stroke volume.

Heart rate is in beats per minute, stroke volume is in cm³ and cardiac output is in cm³ min⁻¹.
It rearranges to heart rate cardiac output ÷ stroke volume and stroke volume = cardiac output ÷ heart rate.
Heart rate 60 ÷ the time of one cardiac cycle in seconds.

Resting value

Resting value about 4.7 dm³ min⁻¹ for an average adult.

Fitness

Fitness fitter people often have a higher cardiac output because the ventricular muscle is thicker and stronger.

Exercise

Exercise cardiac output rises to match the increased metabolic demand of cells.

Units

1 dm³ equals 1000 cm³, so convert before calculating.

Part 4 of 10Control of breathing

Ventilation centres
- the inspiratory and expiratory centres in the medulla oblongata control inhalation and exhalation.

The inspiratory centre sends impulses along motor neurones to the intercostal muscles and diaphragm, which contract.

Chest volume increases, lung pressure falls below atmospheric pressure and air flows in; the expiratory centre is inhibited.

Stretch receptors in the lungs are stimulated as the lungs inflate and send impulses to inhibit the inspiratory centre.

The expiratory centre is no longer inhibited, so the muscles relax, chest volume falls, pressure rises and air flows out.

As the lungs deflate the stretch receptors become inactive, and the next breath can begin.

Exercise response

Exercise response extra CO₂ forms carbonic acid, which dissociates into H⁺ and HCO₃⁻ and lowers blood pH.
Chemoreceptors in the medulla and in the aortic and carotid bodies detect the fall in pH.
The medulla sends more frequent impulses to the intercostal muscles and diaphragm, increasing breathing rate and depth.
More CO₂ is removed, blood pH returns to normal and breathing returns to normal.

Ventilation rate

Ventilation rate the volume of air moved in and out of the lungs per minute, which rises in exercise.

Part 5 of 10Control of heart rate

Control of heart rate

Cardiovascular control centre

Cardiovascular control centre it is in the medulla oblongata and changes the rate at which the SAN fires, in response to impulses from receptors.
Baroreceptors in the aortic and carotid bodies detect high and low blood pressure.
Chemoreceptors detect changes in CO₂, O₂ and pH.

Sympathetic neurones

Sympathetic neurones they release noradrenaline, which makes the SAN fire more often and speeds the heart.
- ('fight or flight')

Parasympathetic neurones

Parasympathetic neurones they release acetylcholine, which makes the SAN fire less often and slows the heart.
- ('rest and digest')

Stimulus	Response
High blood pressure	Parasympathetic impulses release acetylcholine and the heart slows
Low blood pressure	Sympathetic impulses release noradrenaline and the heart speeds up
High O₂, low CO₂, high pH	Parasympathetic impulses slow the heart
Low O₂, high CO₂, low pH (exercise)	Sympathetic impulses speed up the heart

Anticipation
- heart rate rises before exercise because of adrenaline and impulses from the cortex.

Part 6 of 10The role of adrenaline

Fight or flight

Fight or flight in stress, fear or excitement, sympathetic neurones stimulate the adrenal medulla to secrete adrenaline into the blood.

Adrenaline targets

Adrenaline targets it binds to receptors on the SAN, increasing the frequency of excitation and heart rate.
More blood brings more oxygen and glucose to muscle, raising aerobic respiration and energy release.
It stimulates the cardiovascular control centre, increasing sympathetic impulses to the heart.
Vessels to less important organs such as the gut and skin constrict, diverting blood to organs used in the response.
Blood flow to the brain stays constant.

Coordination

Coordination fight or flight is controlled by both nervous and hormonal responses.

Part 7 of 10Practical: effects of exercise with a spirometer

Measurements

Measurements tidal volume, breathing rate, oxygen consumption and minute ventilation.
- (air moved in one normal breath at rest)
Minute ventilation = tidal volume × breathing rate, in dm³ min⁻¹.
Vital capacity is the maximum volume that can be breathed in or out in one breath, and residual volume always stays in the lungs.

Spirometer

Spirometer the subject breathes through a mouthpiece, soda lime absorbs exhaled CO₂ and a rotating drum or computer records a trace.

The subject at rest breathes into the spirometer for one minute and the results are recorded.

The subject exercises for two minutes, for example on a treadmill, while the chamber is refilled with oxygen.

Immediately after exercise the subject breathes into the spirometer for one minute and the results are recorded.

Compare the recordings before and after exercise.

Analysis

Analysis count peaks per minute for breathing rate.
Tidal volume is the average difference between each peak and trough.
Oxygen consumption is the fall in the trace volume over time, because CO₂ is absorbed.

Safety

Safety the subject should be healthy, the mouthpiece disinfected, and exercise stopped if dizziness occurs.

Part 8 of 10Health, medicine and ethics

1Training effects

Training effects heart muscle enlarges (hypertrophy), raising stroke volume and lowering resting heart rate.
Muscle gains more capillaries and mitochondria, and VO₂ max increases.

2Benefits and risks

Benefits and risks moderate exercise lowers the risk of coronary heart disease, type 2 diabetes and obesity and boosts natural killer cell activity.
Excessive exercise reduces immune activity and damages joints, for example anterior cruciate ligament tears and cartilage wear.

3Medicine

Medicine keyhole surgery gives faster recovery and prostheses such as knee replacements and artificial limbs allow people with injuries or disabilities to exercise.

4Performance-enhancing drugs

Performance-enhancing drugs arguments against are unfair advantage, health risks and coercion.
Steroids can cause liver damage and infertility, and EPO thickens blood and risks stroke.
Arguments for are personal choice and a level field if everyone could use them.

Part 9 of 10More from the specification

1Echocardiographic stroke volume assessment

Echocardiographic stroke volume assessment measuring ventricular end-diastolic and end-systolic dimensions provides a non-invasive means to determine cardiac ejection fraction.

2Sliding filament theory

Sliding filament theory myosin heads bind actin, pull the filaments past each other, and the sarcomere shortens.

3ATP role

ATP role it detaches myosin heads from actin and re-cocks them for the next cycle.

4Muscle twitch

Muscle twitch a single contraction followed by relaxation, whereas sustained stimulation gives summation.

5Cardiac muscle

Cardiac muscle it contracts without nervous stimulation and is myogenic, with intercalated discs linking cells.

6Carbon dioxide

Carbon dioxide raised in exercise, it is detected by the medulla and increases heart and breathing rate.

7Filament

A thin high-resistance wire in a lamp that glows when current flows.

8Molecule

Two or more atoms bonded together.

Part 10 of 10More on this topic

Adrenaline
- it increases heart rate and blood flow to skeletal muscles in short-term stress.

Key terms

Term	Meaning
Myogenic	Able to contract without an external nerve stimulus.
Sinoatrial node	The pacemaker in the right atrium that starts each heartbeat.
Purkyne fibres	Conducting fibres that spread the impulse through the ventricle walls.
Cardiac output	Heart rate multiplied by stroke volume.
Tidal volume	The volume of air breathed in or out in one normal breath.
Minute ventilation	Tidal volume multiplied by breathing rate.
Chemoreceptor	A receptor that detects changes in blood chemistry such as CO~2~ or pH.
Baroreceptor	A receptor that detects changes in blood pressure.

Avoid these mistakes

Saying the brain starts each heartbeat — the heart is myogenic and the SAN starts it, while the brain only changes the rate.

Saying the AVN speeds the impulse — it delays it so the ventricles contract after the atria.

Mixing up the nerves — sympathetic neurones speed the heart and parasympathetic neurones slow it.

Forgetting to convert cm³ to dm³ when calculating cardiac output.

Last updated: 18 October 2026