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