Illness was associated with an imbalance of the four humors; with the identity of the excess humor being revealed by the sick person’s symptoms. Consequently, treatment consisted of removing an excess of the identified humor by bloodletting, sweating, or inducing vomiting or diarrhea. Since blood was considered the major humor, practically any disease could be interpreted as occurring from its excess.
By the first century, bloodletting was a common treatment. However, it was the Greek physician Galen (~129 - ~200 CE) who maintained that blood was the most predominant humor. He discovered that both veins and arteries were filled with blood, not air, as was commonly believed at the time. Galen propounded that blood was created but not used up. Since it did not circulate, it would become stagnant in the extremities.
Thus the correlation between bloodletting and balance of the humors. Through the force of his prolific writings, he was able to have an extraordinary influence on medical practice to the effect that bloodletting gained even greater importance and became the standard of treatment for many diseases up through the Middle Ages and beyond.
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