7.3.2Topic 7 · Run for your Life
Electrical Activity in the Heart — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
5 min read 8 parts Lesson 16 of 25 1 quiz
7.3.2 Electrical Activity in the Heart — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe how the heart beat is initiated and coordinated, and how heart rate is controlled by the medulla and autonomic nerves.Cardiac muscle is myogenic: it contracts and relaxes rhythmically w…
- sinoatrial node (SAN)
- atrial systole
- atrioventricular node (AVN)
- bundle of His
- Purkyne fibres
- ventricular systole
Section 1 of 8Myogenic stimulation
Cardiac muscle is myogenic: it contracts and relaxes rhythmically without any external nervous stimulus.
- which acts as the pacemaker.
Section 2 of 8Coordination of the cardiac cycle
The heart must contract in a particular order: both atria first, and then both ventricles from the bottom upwards, so that blood is pushed out through the arteries. The sequence is:
- The AVN delays transmission by about 0.1 second, giving time for the atria to empty completely into the ventricles.
- causing the ventricles to contract from the bottom upwards (ventricular systole) and force blood into the pulmonary artery and aorta.
After this the heart muscle repolarises and relaxes (diastole) until the next wave from the SAN.
Section 3 of 8Controlling heart rate
Although the heart beats by itself, its rate must change with the body's needs. The cardiovascular control centre in the medulla oblongata has two regions: one that increases the heart rate through the sympathetic nerve (accelerator nerve) and one that decreases it through the vagus nerve (parasympathetic).
- A rise in CO2 (lower pH) increases the impulse frequency to the medulla,
- which increases impulses along the sympathetic nerve to the SAN, raising heart rate.
- A fall in CO2 causes the vagus to slow it.
- High pressure causes impulses to the medulla and an increase in vagus activity to slow the heart;
- low pressure causes sympathetic stimulation to speed it up.
Section 4 of 8Heart rate control as a feedback loop
| Part of loop | Example |
|---|---|
| Stimulus | Rise in blood CO2 during exercise |
| Receptor | Chemoreceptors in carotid and aortic bodies |
| Coordinator | Cardiovascular centre in the medulla |
| Effector | SAN and cardiac muscle |
| Response | Greater heart rate and stroke volume; faster removal of CO2 |
As the CO2 level drops back, receptors send fewer impulses and the rate returns to normal, so negative feedback operates. The heart rate and the volume ejected with each beat are what determine cardiac output, which is calculated in a later lesson.
Section 5 of 8Evidence and applications
Evidence that the heart is myogenic is that a heart removed from the body, supplied with oxygenated fluid, continues to beat for some time, and that heart transplants work even though the nerves are cut. After transplant, the heart beats at a faster resting rate (about 100 beats per minute) because the vagus nerve is no longer slowing it, and the rate increases more slowly during exercise because it must rely on circulating adrenaline rather than on nerve impulses.
When the SAN is damaged or the conduction pathway fails, an artificial pacemaker can be implanted under the skin.
- when it detects a missed or slow beat, delivers small electrical pulses through wires to the cardiac muscle to restore a regular rhythm.
- which is why patients with heart block can feel dizzy or faint.
Section 6 of 8Exam-style questions
Explain why there is a delay between atrial and ventricular contraction. [3]3 marks
A band of non-conducting collagen prevents the wave passing directly from the atria to the ventricles. The wave must pass through the AVN, which delays it by about 0.1 s. This allows the atria to finish contracting and empty blood into the ventricles before they contract.
Describe the pathway of the wave of excitation from the SAN to the ventricles. [4]4 marks
SAN to the atrial walls, causing atrial systole; to the AVN; to the bundle of His in the septum; down to the apex and up the walls along Purkyne fibres, causing ventricles to contract from the base upwards.
Explain how exercise causes the heart rate to increase. [4]4 marks
Respiration increases and produces more CO2, which lowers blood pH. Chemoreceptors in the carotid arteries and aorta detect this and send more impulses to the medulla. The medulla sends more impulses via the sympathetic nerve to the SAN, which increases the rate of the heart beat.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Myogenic | Able to contract and relax without nervous stimulation |
| Sinoatrial node | Pacemaker in the right atrium wall that starts each heart beat |
| Atrioventricular node | Node that delays the wave before passing it to the ventricles |
| Bundle of His | Fibres in the septum that carry the wave to the apex |
| Purkyne fibres | Fibres that spread the wave through the ventricular walls |
| Medulla oblongata | Brain region containing the cardiac and ventilation centres |
| Chemoreceptor | Receptor that detects chemical change such as pH or CO2 |
| Baroreceptor | Receptor that detects changes in blood pressure |
Section 8 of 8Exam technique
Marked practiceQuizzes for this lesson
Last updated: