What was "bloodletting" in ancient Europe?

The theoretical basis of bloodletting therapy is from the ancient Greek medical saint Hippocrates and the ancient Roman medical doctor Galen,

Saying that human life depends on four kinds of bodily fluids, blood, mucus, black bile and yellow bile, which correspond to air, water, earth and fire, and China's "gold, wood, water, fire, and earth," which is very close to the Chinese "gold, wood, water, fire, and earth". Ancient Greeks believe that blood in the four kinds of bodily fluids is dominant, Galen that blood is produced by the human body, often "excess", Galen also put the human body subcutaneous arterial and venous blood vessels and the body of the various visceral organs and tubes linked to the so-called "phase of the table", get different diseases, and the "phase of the table" is in the "phase of the table".

For example, the blood from the right arm vein is used to treat liver disease, and the blood from the left arm vein is used to treat spleen disease.