IB Biology SL

Revision Notes

6.2.5 The Heart Rate

Heart Rate: Alteration by Nervous System

  • 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

Location of medulla. (1), downloadable AS & A Level Biology revision notesLocation of medulla. (2), downloadable AS & A Level Biology revision notes

The location of the medulla helps to protect it from harm. It has an essential function as a cardioregulatory centre.

Heart Rate: Alteration by Hormonal System

  • 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!

Author: Ruth

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.

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