Scene of bloodletting, M. B. Valentini, 1715. Bloodletting was based on an ancient system of medicine in which blood and other bodily fluid were regarded as "humors" that had to remain in proper balance to maintain health. It was the most common medical practice performed by physicians from antiquity until the late 19th century, a span of almost 2,000 years. In the overwhelming majority of cases, the historical use of bloodletting was harmful to patients.