control of the heart rate

the heart rate is controlled by the medulla oblongata in the brain, in the medulla oblongata there are two centres, one centre increases the heart rate via the sympathetic nervous system:

  • sends impulses to the sinoatrial node via the accelerator nerve

the other centre decreases the heart rate via the parasympathetic nervous system

  • sends impulses via the vagus nerve

both the accelerator and vagus nerve are motor neurones

the concentration of carbon dioxide in the blood is monitored by receptors called chemoreceptors, we find chemoreceptors in the aorta and the carotid arteries which deliver blood to the brain, and also in the medulla oblongata

increasing heart rate:

  • chemoreceptors monitor the pH of the blood, carbon dioxide dissolves in water in the blood forming carbonic acid - this causes the pH of the blood to decrease

  • the chemoreceptors then send an increased frequency of impulses to the heart rate increase centre in the medulla oblongata

  • the heart rate increase centre sends an increased frequency of impulses to the sinoatrial node causing the heart rate to increase

  • increased blood flow causes more carbon dioxide to leave the blood via lungs and causes concentration of carbon dioxide in the blood to return to normal

decreasing heart rate:

  • pH of the blood now increases - chemoreceptors detect this

  • decreased frequency of impulses sent to heart rate increase centre

  • decreased frequency of impulses to the sinoatrial node

  • returns heart rate back to normal range

if blood pressure changes, this is detected by baroreceptors found in the aorta and carotid arteries

  • blood pressure increases

  • baroreceptors send impulses to the heart rate decrease centre in medulla oblongata

  • medulla oblongata sends impulses via parasympathetic neurones to the sinoatrial node

  • decreasing heart rate and causing blood pressure to fall back to the normal range

under stressful conditions the adrenal glands release the hormones adrenaline and noradrenaline into the bloodstream

both of these hormones causes the sinoatrial node to increase heart rate - by increasing blood flow to muscles more oxygen is provided for muscle contraction