IB Biology SL

Revision Notes

6.2.1 The Blood System: History

Discovery of the Circulation of Blood

Galen’s theories about blood and circulation

  • Galen was a Greek philosopher and surgeon who developed the following theories about blood and circulation
    • Blood is formed in the liver from ingested food
    • Blood is pumped backwards and forwards between the liver and the right ventricle in the heart
    • Some blood moves into the left ventricle through invisible pores and mixes with air from the lungs
    • This mixing of air with blood produces spirits which are distributed to the body via the brain
    • Blood is consumed by the tissues so that new blood must be continuously made

NOS: Theories are regarded as uncertain

William Harvey’s theories

  • William Harvey was an English Physician who overturned Galen’s theories about blood and circulation through a series of experiments and observations
  • Harvey developed the following theories (which were ridiculed at the time
    • Blood is pumped to the brain and body by the heart
    • Blood circulates through the pulmonary and systemic circulations
    • Capillaries exist which link arteries to veins
    • The liver was not capable of producing enough blood to support the idea that blood is consumed by the tissues

Discovery of the Heart as a Pump

  • Harvey used a series of experiments to show that blood flow is unidirectional and that the presence of valves prevents backflow in veins
    • Harvey attached a tourniquet to a persons upper arm and instructed them to grip tightly onto a pole
    • Once the veins became visible, Harvey proceeded to apply pressure to the veins systematically to show how blood flow was affected
    • He used this method to demonstrate how blood moves unidirectionally through the veins in the arm
  • He then demonstrated how the heart acts as a pump which forces blood out through the arteries, it then circulates around the body before it returns to the heart through the veins
  • He also showed that the blood being pumped out of the heart was travelling too quickly to have been produced by the liver (as suggested by Galen)
  • Despite not having advanced enough equipment to visualise the capillaries, Harvey predicted their presence as small vessels which link the arteries to the veins

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