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This lesson covers the cardiac cycle (the sequence of events in one complete heartbeat) and the control of heart rate as required by the Edexcel A-Level Biology specification (9BI0). You need to be able to describe and interpret pressure and volume changes during the cardiac cycle, understand the role of the heart's conduction system, and explain how heart rate is regulated.
The cardiac cycle is the sequence of events that takes place during one complete heartbeat. At a resting heart rate of approximately 72 beats per minute, one cardiac cycle lasts about 0.8 seconds.
The cycle has three main phases:
Key Definition: Systole — the phase of contraction (of either the atria or ventricles). Diastole — the phase of relaxation (of both atria and ventricles simultaneously).
Understanding pressure changes is crucial for interpreting cardiac cycle graphs. The key principle is: valves open when pressure behind them exceeds pressure in front; valves close when pressure in front exceeds pressure behind.
| Time | Event | Pressure changes |
|---|---|---|
| Atrial systole | Left atrium contracts | Atrial pressure rises slightly, exceeding ventricular pressure → blood enters ventricle |
| Ventricular systole begins | Left ventricle contracts | Ventricular pressure rises rapidly |
| AV valve closes | Ventricular pressure > atrial pressure | First heart sound ("lub") |
| Semilunar valve opens | Ventricular pressure > aortic pressure | Blood ejected into aorta |
| Semilunar valve closes | Ventricular pressure falls below aortic pressure | Second heart sound ("dub"); small "notch" in aortic pressure trace |
| Diastole | Ventricle relaxes | Ventricular pressure drops; AV valve opens when ventricular pressure < atrial pressure |
Exam Tip: When reading a cardiac cycle graph, identify valve events by looking for where pressure lines cross. The AV valve closes when the ventricular line rises above the atrial line; the semilunar valve opens when the ventricular line rises above the arterial line. Practise reading these graphs — they are a very common exam question.
The volume of blood in the ventricle changes throughout the cycle:
| Phase | Ventricular volume change |
|---|---|
| Atrial systole | Increases slightly (atrial contraction pushes remaining blood into ventricle) |
| Ventricular systole | Decreases rapidly (blood is ejected into the arteries) |
| Diastole | Increases steadily (ventricle refills passively from the veins) |
Stroke volume = the volume of blood pumped from one ventricle in one contraction = end-diastolic volume – end-systolic volume (typically about 70 ml at rest).
The heart is myogenic — it generates its own rhythmic electrical impulses without external nervous stimulation. The conduction system coordinates the contraction of the heart chambers.
| Structure | Location | Function |
|---|---|---|
| Sinoatrial node (SAN) | Wall of the right atrium, near the superior vena cava | Pacemaker — initiates the electrical impulse that triggers each heartbeat |
| Atrioventricular node (AVN) | At the junction of the atria and ventricles (in the septum) | Receives the impulse from the SAN; introduces a brief delay (~0.1 s) to allow the atria to finish contracting before the ventricles begin |
| Bundle of His | In the interventricular septum | Conducts the impulse rapidly from the AVN down the septum towards the apex of the heart |
| Purkinje fibres | Throughout the ventricular walls, branching from the Bundle of His | Distribute the impulse rapidly through the ventricular muscle, causing contraction from the apex upwards (which efficiently pushes blood upwards into the arteries) |
Exam Tip: The delay at the AVN is critical. Without it, the atria and ventricles would contract simultaneously, and the atria would not have time to empty into the ventricles. Always mention this delay when describing the conduction system.
An electrocardiogram (ECG) is a recording of the electrical activity of the heart, detected by electrodes placed on the skin.
A normal ECG has three main features:
| Wave | Represents |
|---|---|
| P wave | Depolarisation (contraction) of the atria — atrial systole |
| QRS complex | Depolarisation (contraction) of the ventricles — ventricular systole. The QRS is larger than the P wave because the ventricles have more muscle mass. |
| T wave | Repolarisation (relaxation) of the ventricles |
Note: Atrial repolarisation is hidden within the QRS complex.
| Condition | ECG pattern |
|---|---|
| Normal sinus rhythm | Regular P waves followed by QRS complexes at ~72 bpm |
| Tachycardia | Normal pattern but at an elevated rate (>100 bpm) |
| Bradycardia | Normal pattern but at a reduced rate (<60 bpm) |
| Atrial fibrillation | No clear P waves; irregular QRS complexes |
| Ventricular fibrillation | No recognisable waves; chaotic pattern — a medical emergency |
| Heart block | P waves not followed by QRS complexes (the impulse is not transmitted from atria to ventricles) |
Exam Tip: You may be asked to calculate the heart rate from an ECG. Count the number of complete cardiac cycles (P-QRS-T) in a given time interval, or measure the distance between two successive R peaks and use: heart rate (bpm) = 60 / (time between R peaks in seconds).
Although the heart is myogenic, its rate is adjusted by the autonomic nervous system and hormones in response to the body's needs.
The cardiovascular centre (CVC) is located in the medulla oblongata of the brain. It receives input from sensory receptors and sends signals to the SAN via two nerves:
| Nerve | Effect on heart rate |
|---|---|
| Sympathetic nerve (accelerator) | Releases noradrenaline at the SAN → increases heart rate and force of contraction |
| Parasympathetic nerve (vagus nerve) (decelerator) | Releases acetylcholine at the SAN → decreases heart rate |
| Receptor | Stimulus | Response |
|---|---|---|
| Chemoreceptors (in carotid and aortic bodies, and in the medulla) | Rise in blood CO₂ concentration (or fall in pH) | Sympathetic stimulation → heart rate increases |
| Baroreceptors (pressure receptors) (in carotid sinus and aortic arch) | Rise in blood pressure | Parasympathetic stimulation via vagus nerve → heart rate decreases |
| Baroreceptors | Fall in blood pressure | Sympathetic stimulation → heart rate increases |
| Stretch receptors in muscles | Movement/exercise | Sympathetic stimulation → heart rate increases |
Exam Tip: When explaining the control of heart rate, always include: (1) the stimulus detected, (2) the receptor, (3) the nerve/hormonal pathway, (4) the effector (SAN), and (5) the response (increase or decrease in heart rate). This is a classic example of a negative feedback loop.
Cardiac output (CO) is the total volume of blood pumped by one ventricle per minute.
Cardiac output = stroke volume × heart rate
CO = SV × HR
| Term | Definition | Typical resting value |
|---|---|---|
| Stroke volume (SV) | Volume of blood pumped per beat | ~70 ml |
| Heart rate (HR) | Number of beats per minute | ~72 bpm |
| Cardiac output (CO) | Volume of blood per minute | ~5,000 ml/min (5 litres/min) |
During exercise, both HR and SV increase, so cardiac output can rise to 20–25 litres per minute in a trained athlete.
| Feature | Detail |
|---|---|
| Cardiac cycle | Atrial systole → ventricular systole → diastole (~0.8 s total) |
| Pacemaker | Sinoatrial node (SAN) |
| Delay | Atrioventricular node (AVN) — 0.1 s |
| ECG waves | P (atrial depolarisation), QRS (ventricular depolarisation), T (ventricular repolarisation) |
| Heart rate control | CVC in medulla; sympathetic (increases HR), parasympathetic/vagus (decreases HR) |
| Cardiac output | CO = SV × HR |
Mastering the cardiac cycle, ECG interpretation, and heart rate control is essential — these topics combine physiology, graph interpretation, and the application of feedback principles.
This material sits at the dynamic core of Edexcel 9BI0 Topic 7 (Run for your life — Exchange and Transport). Where lesson 4 fixed the gross anatomy of the four-chambered heart, this lesson turns that anatomy into a time-resolved sequence: atrial systole, ventricular systole and diastole, generated by the SAN-AVN-Bundle of His-Purkyne pathway and modulated by the autonomic nervous system. Statements expect candidates to interpret pressure-time and volume-time diagrams, justify each valve event from chamber-pressure inequalities, label and interpret a normal ECG (P, QRS, T), and explain heart-rate control by the medullary cardiovascular centre via sympathetic and parasympathetic (vagal) outputs. The material is synoptic with lesson 6 (blood vessels) — the systolic pulse generated here is what arterial elastic walls absorb; with lesson 7 (haemoglobin) — peak ejection pressure sets the rate of erythrocyte delivery to tissues; with Topic 8 (Grey Matter) — autonomic outputs reach the SAN through medullary baroreceptor and chemoreceptor reflexes; and with Topic 5 (Energy for Biological Processes) — cardiac output rises during exercise to match cellular respiration's demand. Examiners pair anatomical-functional recall (AO1) with quantitative cardiac-output and graph-reading work (AO2) and stretch synthesis on negative-feedback control (AO3). Refer to the official Pearson Edexcel 9BI0 specification document for the exact wording of the relevant statements.
Question (8 marks):
Figure 1 (not shown) is a pressure-time diagram of the left side of the heart over one cardiac cycle, showing left-atrial, left-ventricular and aortic pressure traces.
(a) At point X, atrial pressure is 8 mmHg, ventricular pressure is 6 mmHg and aortic pressure is 80 mmHg. State, with reasons, the state (open/closed) of the bicuspid valve and the aortic semilunar valve at point X. (2)
(b) At point Y, ventricular pressure has risen to 110 mmHg, atrial pressure is 10 mmHg and aortic pressure is 95 mmHg. State the state of each valve at point Y and name the phase of the cardiac cycle. (3)
(c) The "lub-dub" of a stethoscope corresponds to two distinct heart sounds. Explain, in terms of valve events, the cause of each sound and the cardiac-cycle phase in which it occurs. (3)
Solution with mark scheme:
(a) M1 (AO2) — The bicuspid (AV) valve is open because atrial pressure (8 mmHg) exceeds ventricular pressure (6 mmHg); blood flows from atrium into ventricle.
A1 (AO2) — The aortic semilunar valve is closed because aortic pressure (80 mmHg) far exceeds ventricular pressure (6 mmHg). Point X is therefore late diastole / early atrial systole.
A common pitfall is naming the valve states without naming the pressure inequalities — examiners reward the because, not just the answer.
(b) M1 (AO1) — Phase: ventricular systole.
M1 (AO2) — Bicuspid valve closed: ventricular pressure (110 mmHg) exceeds atrial pressure (10 mmHg).
A1 (AO2) — Aortic semilunar valve open: ventricular pressure (110 mmHg) exceeds aortic pressure (95 mmHg), so blood is being ejected into the aorta.
(c) M1 (AO1) — The first heart sound ("lub") is produced by closure of the atrioventricular valves (bicuspid and tricuspid).
M1 (AO2) — It occurs at the start of ventricular systole, when ventricular pressure rises rapidly to exceed atrial pressure, slamming the AV valve flaps shut against the chordae tendineae.
A1 (AO2) — The second heart sound ("dub") is produced by closure of the semilunar valves (aortic and pulmonary) at the start of diastole, when ventricular pressure falls below arterial pressure and the brief reverse flow snaps the pocket cusps shut.
Total: 8 marks (M5 A3).
Question (6 marks): Explain how the cardiovascular centre in the medulla oblongata adjusts heart rate in response to a rise in blood carbon dioxide concentration during exercise.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Identifying that chemoreceptors in the carotid bodies, aortic bodies and the medulla detect a rise in blood CO2 (or fall in pH) |
| 2 | AO1.2 | Stating that the cardiovascular centre in the medulla oblongata receives this sensory input |
| 3 | AO2.1 | Describing increased frequency of action potentials in the sympathetic nerve to the SAN |
| 4 | AO2.1 | Stating that noradrenaline is released at the SAN, increasing its rate of depolarisation and therefore heart rate |
| 5 | AO2.7 | Linking raised heart rate (and therefore raised cardiac output) to faster removal of CO2 at the lungs and faster delivery of O2 to respiring muscle |
| 6 | AO3.1 | Synthesis: this is a negative-feedback loop — the increase in heart rate restores blood CO2 towards the set point, and chemoreceptor firing decreases as a result |
Total: 6 marks (AO1 = 2, AO2 = 3, AO3 = 1). Edexcel control-of-heart-rate questions of this type reliably split AO marks roughly 30/50/20 across AO1/AO2/AO3.
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