Dr. Ian Halim: The Yin and Yang of Bloodletting

.....Somerville Bagel Bards member and physician-humanist, Ian Halim, writes about how medicine relates to everything from ethics to botany—aiming to make science accessible to a wide and varied audience. This article will also appear in The Somerville Times.

For centuries, doctors would drain their patients’ blood on purpose in order to try to help them. The ancient but misguided theory was that the body consisted of different fluids, and that health required the right balance of these fluids—humors, as they used to be called. These humors were believed to be responsible not only for health, but even personality and temperament. We get the phrases “ill humor” and “sense of humor,” from this old idea. Black bile was one such humor and gives us the modern word melancholy for sadness (from the Greek “black” melan- and “bile” chol-). Removing blood from the sick was a wrongheaded way to try to correct the supposedly-disease-causing imbalance in the different humors.

This was a theory without evidence. In reality, bloodletting is an assault on the body, starving it of nutrients and the power to supply the tissues with oxygen. When a patient is already sick, bleeding them makes them sicker—and more likely to succumb to disease.

These days, we know that controlled bloodletting is only a good treatment for a couple of rare diseases.


Polycythemia vera is a rare disease of red blood cell excess often caused by a genetic mutation. The treatment, intuitively enough, is removing these excess red blood cells—a kind of modern bloodletting. This is done by using a syringe to draw off blood. As usual, the term for this is from Greek—from words meaning “vein” (phleb-) “cutting” (tom-). In modern hospitals, phlebotomy also refers (in a much more general sense) to the team that handles all blood draws. When you order laboratory studies for a patient, they are added to the “phlebotomy queue.”

Hemochromatosis is another disease treated by phlebotomy. It can result from an inherited problem in the way our bodies process iron, and it's a disease not of red blood cell excess per se, but of iron overload. Iron is a necessary ingredient in the hemoglobin pigment that colors our red blood cells and allows them to carry oxygen. But too much iron is not good. As the saying goes, the dose makes the poison. Over time, if hemochromatosis is untreated it can cause liver failure, diabetes, heart failure, arthritis, and a telltale bronzing of the skin. One of the main places that our body stores iron is in our red blood cells, making removing blood via phlebotomy the best, simplest way to lower the body’s stores of iron and treat the disease.

Except for polycythemia and hemochromatosis, though, losing more than a little blood is usually harmful.




Nowadays, doctors are well practiced at tracking the amount and rate of blood loss. Blood pressure, heart rate, hemoglobin levels, and hematocrit levels are all invaluable ways to track blood loss.




Imagine a balloon filled with water, taut and stretched out. The pressure on it is great. If it is drained, the pressure will drop, and the surfaces will sag and tent inwards. This is a rough approximation of what happens in blood loss. The body can compensate by narrowing blood vessels and using the heart to pump the blood harder and faster. But eventually, if the blood loss is too rapid or too great, these compensatory strategies will fail and the blood pressure will start to drop.




When a patient comes into the emergency room after suffering a car crash or a stab wound, or after starting to vomit blood, having a way to track blood loss is critical. Nearly the first thing that happens when a patient like this comes into the hospital is their vital signs are taking—temperature, heart rate, blood pressure, and the oxygen saturation of their blood. A drop in blood pressure with an increase in heart rate is classic for blood loss (although not unique to blood loss), and will immediately raise concern about the patient, depending on how much the blood pressure has dropped and how much the heart rate has risen. As the balloon grows flaccid—in our model—the heart pumps harder and faster to counteract this. Without adequate blood pressure, tissues will not be getting enough oxygen-rich blood. And eventually, if too much blood is lost, no matter how hard the heart works to counteract it, the blood pressure will continue to drop.




Another way that doctors monitor blood loss is by measuring the hemoglobin and hematocrit—H &H for short. When someone is actively bleeding, the H&H might be taken at regular intervals, to monitor the rate of blood loss. The hematocrit is the relative volume of red blood cells within a given amount of blood. Hemoglobin is the metal protein complex that transports oxygen within our red blood cells, making up the bulk of their non-water weight. If the body loses blood, it will try to retain fluid to maintain total blood volume, but it may not be able to keep up with making enough red blood cells to replace those that are lost—resulting in a relatively dilute blood, and therefore lower amounts of hemoglobin and hematocrit in a given volume of blood.




Hemoglobin, hematocrit, blood pressure, and heart rate are all very useful ways for doctors to monitor blood loss. The clinical situation is very important too, of course, and the idea is to figure out why someone is losing blood, and to stop it. The urgency of intervening will often depend upon the rate of blood loss, and the total amount of blood loss, which can be estimated with our four objective measures. These objective measures are especially meaningful when combined with clinical information about what’s happened to the patient, since other things can cause a high heart rate and a low blood pressure, for example, such as an infection.




These days, we can set aside the theory of the humors. We know that our red, oxygen-transporting blood is precious life-giving stuff. And we are often in a pretty good position to make sure that someone has enough of it to stay alive.