6.2.5 The Heart Rate

• Although the heart muscle maintains a base heart rate via myogenic stimulation, there are several circumstances that can cause an individual's heart rate to change, e.g.
  • Exercise
  • Stress
  • Relaxation

• The brain is involved in the regulation of heart rate, though it does not require conscious thought
  • The branch of the nervous system that does not require conscious thought is known as the autonomic nervous system

• The area of the brain that controls heart rate is the cardiovascular centre, located in a region of the brain called the medulla
• The medulla is found at the base of the brain near the top of the spinal cord
• Two nerves connect the medulla with the sinoatrial node (SAN):
  • One nerve connects to the acceleratory centre, which causes the heart to speed up
    • This happens in response to low blood pressure, low oxygen concentrations and low pH
    • These changes might occur during exercise
    • The blood vessels dilate, causing a decrease in blood pressure
    • The muscle cells are using up oxygen at a faster rate, causing blood oxygen levels to drop
    • The production of carbon dioxide by respiring cells causes blood pH to decrease

  • The other nerve connects to the inhibitory centre, which causes the heart to slow down
    • This happens in response to high blood pressure, high oxygen concentrations and high pH
    • These changes are likely to occur when the body is at rest

The heart rate is controlled by the cardiovascular centre in the medulla

• Epinephrine, also called adrenaline, is produced by the adrenal glands, located above the kidneys, in times of fear, stress, or excitement
• The brain controls the release of epinephrine from the adrenal glands
• Epinephrine increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’
  • Increased glucose and oxygen are needed by the cells for aerobic respiration to release energy, e.g. to fuel the muscles to move/run away!