How did William Harvey’s findings on blood circulation challenge earlier theories, such as those of Galen?

Sourav PanNovember 8, 2024

Answered

William Harvey’s findings on blood circulation marked a significant departure from earlier theories, particularly those proposed by the ancient Greek physician Galen. Here’s how Harvey’s discoveries challenged Galen’s views:

1. Galen’s Theories of Blood Circulation

• Production of Blood: Galen posited that blood was produced in the liver from ingested food and then distributed throughout the body. He believed that blood flowed from the liver to the heart, where it was mixed with air (pneuma) absorbed from the lungs, and then sent out to the body via arteries.
• Open-Ended System: Galen described a system where blood did not circulate in a closed loop but rather ebbed and flowed. He theorized that blood passed between the heart’s ventricles through invisible pores in the interventricular septum and that blood dissipated at the ends of veins and arteries according to local tissue needs. This view held sway for over 1,400 years.

2. Harvey’s Discoveries

• Closed Circulatory System: In his seminal work De Motu Cordis (On the Motion of the Heart and Blood), published in 1628, Harvey demonstrated that blood circulates continuously in a closed system of arteries and veins. He argued that blood is pumped by the heart, not produced by the liver or dissipated into tissues.
• Role of Valves: Harvey’s examination of venous valves led him to conclude that they prevent backflow, ensuring that blood flows toward the heart rather than away from it. This contradicted Galen’s belief that blood could flow freely in both directions.
• Quantitative Evidence: Harvey estimated the volume of blood pumped by the heart and calculated that it was impossible for the body to produce or consume blood at the rates suggested by Galen’s model. For instance, he calculated that if Galen’s theory were correct, the liver would have to produce an implausible amount of blood (up to 500 pounds per day) to account for circulation.

3. Impact on Medical Understanding

• Shift in Paradigm: Harvey’s hydraulic model of circulation replaced Galen’s more abstract notions about blood movement with a mechanical understanding based on direct observation and experimentation. He demonstrated that blood flows from the heart through arteries to tissues and returns via veins, establishing a clear distinction between systemic and pulmonary circulation.
• Foundation for Modern Physiology: Harvey’s findings laid the groundwork for modern cardiovascular physiology, influencing how subsequent generations understood blood flow, heart function, and vascular health.