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1.1.5Topic 1 · Lifestyle, Health and Risk

Cardiac Cycle — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 7 of 23 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Lesson summary

1.1.5 Cardiac Cycle — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the three stages of the cardiac cycle and interpret pressure and volume data, including how the cycle is initiated and controlled.The cardiac cycle is the sequence of contraction (systole) and relaxation …

  • cardiac output
  • Atrial systole
  • Ventricular systole
  • electrocardiogram (ECG)
  • QRS complex
  • tachycardia

Part 1 of 6What is the cardiac cycle?

The cardiac cycle is the sequence of contraction (systole) and relaxation (diastole) of the heart chambers that makes up one heartbeat. At a resting heart rate of 70 beats per minute one cycle lasts about 0.85 s. Pressure changes in the chambers cause the valves to open and close, so that blood flows in one direction.

The cardiac output is the volume of blood pumped per minute:

cardiac output = heart rate x stroke volume

For example, 70 beats per minute x 75 cm3 per beat = 5250 cm3 per minute (about 5.3 dm3).

Part 2 of 6Stages

1
Atrial systole.
  • The atria contract, raising the atrial pressure above ventricular pressure.
  • The atrioventricular valves are pushed open and blood is forced into the ventricles, topping up the volume.
  • The semilunar valves remain closed.
2
Ventricular systole.
The ventricles contract from the base upwards.
  • Ventricular pressure quickly exceeds atrial pressure, so the AV valves snap shut (the first heart sound, 'lub').
  • When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected.
  • Aortic pressure reaches around 120 mmHg in a healthy adult.
3
Diastole.
  • The ventricles relax, pressure falls below that in the arteries, and blood in the arteries tries to flow back and closes the semilunar valves.
    • (the second sound, 'dub')
  • Atria fill with blood from the veins.
  • When ventricular pressure falls below atrial pressure the AV valves open and blood flows passively into the ventricles.
  • The elastic recoil of the aorta keeps pressure and flow going.

Part 3 of 6Reading pressure graphs

On a combined graph of pressure in the atrium, ventricle and aorta against time:

1
When the ventricular pressure line crosses above the atrial line, the AV valve closes.
2
When the ventricular line crosses above the aortic line, the semilunar valve opens and the ventricle empties.
3
When the ventricular line falls below the aortic line, the semilunar valve closes, producing a small dip (the dicrotic notch) in the aortic trace.
4
When the ventricular line falls below the atrial line, the AV valve opens.

The time of ventricular systole is found by measuring along the time axis between the AV valve closing and the semilunar valve closing. To work out the heart rate, divide 60 by the length of one cycle in seconds. Always quote values from the graph with units.

Part 4 of 6Heart rate, ECG and measuring cardiac function

An electrocardiogram (ECG) records the electrical activity of the heart through electrodes on the skin.

Heart rate, ECG and measuring cardiac function
P wave
A normal trace has a small P wave (atrial depolarisation), a large QRS complex (ventricular depolarisation, with atrial repolarisation hidden within it) and a T wave (ventricular repolarisation).
Tachycardia
Abnormal patterns indicate problems: tachycardia is a resting rate over 100 beats per minute, bradycardia is below 60,
  • and fibrillation is uncoordinated, rapid contraction of the muscle that fails to pump blood effectively.
Ectopic beats are extra beats initiated outside the SAN.

To calculate heart rate from an ECG, measure the time for one cycle (for example the R-R interval) and divide 60 by it.

Heart rate, ECG and measuring cardiac function
If the interval is 0.75 s the rate is 60 / 0.75 = 80 beats per minute.
Exam questions will often give a trace at a known scale such as 25 mm per second; make sure you convert distances to time correctly.

Part 5 of 6Initiating and controlling the heartbeat

Cardiac muscle is myogenic: it contracts without nervous stimulation.

Initiating and controlling the heartbeat
Sinoatrial node (SAN)
The beat begins in the sinoatrial node (SAN), a group of cells in the wall of the right atrium that acts as the pacemaker.
A wave of depolarisation spreads across both atria, causing atrial systole.
A layer of non-conducting fibrous tissue stops the wave from passing directly to the ventricles.
Atrioventricular node (AVN)
The wave reaches the atrioventricular node (AVN), which delays it by about 0.1 s so atria can finish emptying.
Bundle of His
It then passes down the bundle of His in the septum and spreads up through Purkyne fibres in the ventricle walls,
  • so that ventricles contract from the apex upwards, pushing blood towards the arteries.

The rate is adjusted by the cardiac centre in the medulla oblongata. The sympathetic nerve speeds the SAN; the parasympathetic (vagus) nerve slows it. Chemoreceptors detect carbon dioxide, which lowers blood pH, and baroreceptors detect pressure.

Part 6 of 6Exam-style questions

  1. Describe how the heartbeat is initiated and coordinated. [5]5 marks

  2. A person has a stroke volume of 80 cm3 and a heart rate of 65 beats per minute. Calculate cardiac output in dm3 per minute. [2]2 marks

  3. Explain why the atrioventricular valves close at the start of ventricular systole. [2]2 marks

Key terms

TermMeaning
SystoleThe contraction phase of the cardiac cycle.
DiastoleThe relaxation phase of the cardiac cycle.
Cardiac outputVolume of blood pumped by a ventricle per minute.
Stroke volumeVolume of blood pumped by a ventricle in one beat.
MyogenicContracting in response to its own stimulus, not a nerve impulse.
Sinoatrial nodeThe heart's pacemaker, in the right atrium wall.
Atrioventricular nodeTissue that delays the impulse before it passes to the ventricles.
Purkyne fibresConducting fibres that carry the impulse through the ventricle walls.

Avoid these mistakes

1
Saying the SAN is in the ventricle. It is in the wall of the right atrium.
2
Mixing up systole (contraction) and diastole (relaxation).
3
Saying that the heartbeat begins with a nerve impulse from the brain. It is myogenic; nerves only alter the rate.
4
Forgetting the AVN delay.
5
Writing that valves open and close actively. They respond passively to pressure differences.

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