3.6.1.3 Control of heart rate

Question 1

Describe how a heartbeat is initiated and coordinated (5 marks)

Answer 1

1. Sinoatrial node (SAN) sends a wave of electrical impulses across atria, causing atrial contraction
2. Non-conducting tissue prevents immediate contraction of ventricles/ prevents impulses reaching ventricles
3. Atrioventricular node (AVN) delays impulse whilst blood leaves atria and ventricles fill
4. AVN sends impulse down Bundle of His
5. Causes ventricles to contract from base up

Question 2

Explain what causes heart rate to increase during exercise (6 marks)

Answer 2

1. Rate of respiration in muscles cells increases
2. Carbon dioxide concentration increases and blood pH decreases
3. Chemoreceptors in aortic/ carotid bodies
4. Sends impulses to the medulla (specifically the cardioaccelerator centre)
5. Increases frequency of impulses
6. Along sympathetic pathway to sinoatrial node

Question 3

Describe the myogenic stimulation of the heart and how the regular contraction of the atria and ventricles is coordinated (5 marks)

Answer 3

1. SAN releases wave of electrical activity
2. Atria contracts
3. AVN passes electrical activity after a short delay
4. Via Purkyne fibres/ Bundle of His
5. Ventricles contract from bottom upwards

Question 4

Explain how a rise in blood pressure results in a decrease in rate of heartbeat (6 marks)

Answer 4

1. Pressure receptors/ baroreceptors
2. Sends impulses to cardiovascular centre (medulla)
3. Impulses are sent via parasympathetic nerves
4. Impulses sent to SAN
5. Decreases impulses from SAN
6. Decreases impulses to and from AVN

Question 5

Describe the role of receptors and of the nervous system during exercise in increasing heart rate. (4 marks)

Answer 5

1. Chemoreceptors detect a rise in carbon dioxide/ decrease in pH
2. Sends impulses to cardiac centre/ medulla
3. Increases impulses to SAN
4. By sympathetic nervous system