5.1.5 (k)

Question 1

How is heart rate controlled?

Answer 1

• Heart contraction is involuntary and controlled by the autonomic nervous system
• In the medulla oblongata there are 2 centres which are linked to the SAN to control heartbeat
  
  • Cardiac Inhibitory Centre
    
    • Which sends impulses through the parasympathetic nervous system to the vagus nerve (decelerator) to release Ach (reducing heart rate)
  • Cardiac Accelerator Centre
    
    • Which sends impulses through the sympathetic nervous system to the accelerator nerve releasing noradrenaline to increase heart rate

Question 2

What is the role of a chemoreceptor?

Answer 2

• Present in the aorta, carotid artery (artery in neck providing blood to brain) and medulla
• Sensitive to changes in pH
  
  • As carbon dioxide levels rise, pH falls as carbon dioxide reacts with water forming carbonic acid, which dissociates and releases protons

Question 3

Explain how a chemoreceptor works

Answer 3

• If chemoreceptors detect low blood pH, a response triggers increasing heart rate to increase blood flow to lungs to exhale carbon dioxide
• If chemoreceptors detect high blood pH, (when CO₂ levels are low, fewer nerve impulses are sent to the medulla oblongata, so the frequency of impulses sent to the SAN (via sympathetic nervous system) are fewer, so heart rate returns to normal

Question 5

What is the role of a baroreceptor?

Answer 5

• They are blood pressure receptors and are present in aorta, vena cava and carotid arteries

Question 6

Explain the role of a baroreceptor

Answer 6

• If blood pressure is too high, impulses are sent to the medulla oblongata centre that lowers heart rate
  
  • Impulses are sent along parasympathetic neurones to SAN decreasing rate that heart beats reducing blood pressure to normal
• If blood pressure is too low, impulses are sent to the medulla oblongata centre that increases heart rate
• Impulses are sent along sympathetic neurones to SAN to increase heart rate

Question 7

What is the effect of hormones on heart rate?

Answer 7

• In times of stress, adrenaline and noradrenaline are released to affect the pacemaker region of the heart itself to speed up heart rate
  
  1. By increasing the frequency of impulses produced by the SAN