How did William Harvey challenge Galen’s ideas about blood?

William Harvey challenged the long-standing physiological theories of the Greek physician Galen by demonstrating that blood circulates throughout the body in a single, continuous system, rather than being consumed and produced at the same rate.1

Key aspects of Harvey’s challenge included:

• Circulation vs. Consumption: Galen taught that the liver produced blood from nutrients, which was then consumed by the body's tissues. Harvey used quantitative experiments to show that the heart pumps more blood per minute than the body could possibly produce or consume, proving the blood must circulate.1
• Heart Function: Galen believed the heart's active phase (contraction) drew blood into itself. Harvey observed that the heart's active phase is when it decreases in volume, acting as a pump to expel blood with force.1
• Venous Valves: While Galen’s followers believed valves in the veins prevented blood from pooling due to gravity, Harvey demonstrated that these valves are oriented to inhibit flow away from the heart, supporting the concept of unidirectional circulation.1
• Experimental Method: Harvey rejected the reliance on ancient authority, instead using precise observation and experimentation to establish his findings.2