6.2.1 The Blood System: History

NOS: Theories are regarded as uncertain; William Harvey overturned theories developed by the ancient Greek philosopher Galen on movement of blood in the body

• A theory can be defined as:
  • A carefully thought-out idea, with accompanying evidence, that explains observations of the natural world

• Theories are often constructed using the scientific method which involves bringing together many facts and hypotheses
  • There is always a level of uncertainty when using scientific methods
  • Uncertainty can be due to
    • Natural variability of individual organisms
    • Accuracy of measurements taken

• Theories can therefore be regarded as uncertain due to the uncertainties in the methods used
• New technology or the discovery of new evidence often results in theories being falsified or overturned
• An example of the falsification of a set of theories is that of Galen's theories about the blood and circulation
  • Galen was an ancient Greek philosopher and surgeon who developed the following theories
    • 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

  • Galen failed to present any evidence for his theories

• Galen's theories were overturned by English physician William Harvey 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 circulation systems
    • Capillaries exist which link arteries to veins
    • Blood flow is too fast for blood to be consumed by the tissues; it would run out too quickly to be replaced. Instead, blood returns to the heart and re-circulates

  • Harvey refused to accept Galen's theories without direct evidence and he toured Europe to demonstrate evidence for his own theories to others, eventually leading to acceptance of his new theories

Galen proposed a model which was later disproved by William Harvey

• 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 person's upper arm and instructed them to grip tightly onto a pole
    • A tourniquet is a band applied to a limb to limit blood flow to the lower part of that limb

  • 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

William Harvey showed the movement of blood into the veins of the arms in this simple experiment

• 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 be constantly used up by the tissues, as described by Galen
• Despite not having powerful enough microscopes to see the capillaries, Harvey predicted their presence as small vessels which link the arteries to the veins