Bloodletting was a historically practiced medical procedure which involved removing a set amount of blood from the veins of a patient for therapeutic purposes. It is still retained, in an abstract sense, in the form of phlebotomy, the drawing of blood for the purposes of analysis. In addition to removing blood for analysis, medical personnel may also take blood from a patient for the purpose of blood donation.
Historically, physicians believed that many illnesses were caused by an excess of blood, and bloodletting was a frequent prescription for a wide range of conditions. In some cases, the prescription might have actually been helpful, albeit in a limited way; it would lower blood pressure, for example, by lowering blood volume. However, a large amount of blood loss could potentially make a patient even sicker; unfortunately, many prescriptions for bloodletting recommended repeated sessions if the patient didn't show improvement.
Therapeutic bloodletting was accomplished in a number of ways. Veins could be punctured with knives or needles, for example, and leeches should also be used to suck blood from a patient. Leeches are still used in modern medical treatment to treat specific conditions, such as poor circulation. In some cases, leeches can actually restore the flow of blood to a damaged extremity, potentially preventing the loss of that extremity.
Modern phlebotomy aims to remove a minimal amount of blood through a needle inserted into the vein. This process is also called venesection, venipuncture, or simply blood drawing. A variety of medical personnel are trained in phlebotomy, and the modern practice of bloodletting is a far cry from ancient medicine. Once blood has been drawn, it can be studied for signs of disease, or to monitor a patient's condition.
In the case of blood donation, people can get a sense of what therapeutic bloodletting might have been like. However, most blood donors donate less blood than bloodletting would have removed, and they do it in much safer and more sterile conditions. In addition, blood donors are screened to ensure that they can safely give blood, and people with conditions like anemia, colds, and flu are specifically excluded.
By the 20th century, therapeutic bloodletting had been essentially abandoned, although it is used for a small number of conditions. When bloodletting is prescribed, it is carried out in a sterile environment using tools which have been designed to reduce pain and the risk of infection. The transition from the widespread practice of bloodletting to essential abandonment was part of a larger series of radical changes in the medical world which were the result of a better understanding of medical conditions.