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By Dr. Ian Halim

What happens to our bodies when we bleed? Looking at how we’ve answered this question across time gives us a way to chart the progress of medicine as a science. And in hospitals throughout the world, this question is not history, but a daily matter of life and death.

Since ancient times, healers would drain their patients’ blood on purpose in order to try to help them. The 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 for 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 for “black” melan- “bile” chol-).

Even though it was wrongheaded, there was a kind of logic behind blood-letting. Blood is hot and wet. A person who is sick and feverish and sweating is also hot and wet. This made it more plausible to think of blood-letting as removing the hot and wet humor in order to counteract alarming fever and inflammation. But this plausibility didn’t make blood-letting any less dangerous. In reality, blood-letting 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.

There are other reasons why blood-letting lasted for so long: there were few treatments that were any better, nobody was measuring whether it worked, and doctors thought that the body constantly generated and used up blood – making blood seem less scarce and precious than it really is.

It wasn’t until 1628 that an English doctor figured out that our blood flows through our bodies in a continuous circuit, getting recycled over and over. And now we know that it normally takes about 120 days for the body to replace a red blood cell – far too slow to make up for a fast bleed. So controlled bloodletting is now only used for a couple of rare diseases.

One of these, polycythemia vera, is an excess of red blood cells often caused by a genetic mutation. The treatment, intuitively enough, is removing these excess red blood cells – a kind of modern blood-letting. 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, we now know that losing more than a little blood is usually harmful. And nowadays doctors are well practiced at estimating the amount and speed of blood loss. We can start by considering blood pressure and heart rate.

Imagine a balloon filled with water, taut and stretched out. The pressure on it is great. If it’s 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 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 is the vital signs are taken – temperature, heart rate, blood pressure, and the oxygen level in the blood. An increase in heart rate followed by a dip in blood pressure is classic for blood loss (although these changes are not unique to blood loss). This pattern 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. And eventually, if too much blood is lost, no matter how hard the heart works to counteract it, the blood pressure will keep dropping. Without enough blood pressure, the tissues will be starved of oxygen-rich blood.

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 may be taken at regular intervals, to track 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.

When there is a brisk bleed, at first there will be a loss of total blood volume. Since all the components of blood are lost in roughly equal proportion, the fractions of hemoglobin and hematocrit in a sample of blood will appear normal. This is called volume contraction. But as the body continues to lose blood, it may not be able to make enough red blood cells to keep up. The body will retain fluid to try to maintain the same total blood volume, even as red blood cells are lost. Over time, as the body loses blood but retains fluid, there will be fewer and fewer red blood cells in a given sample of blood. This progressive dilution shows up in declining hemoglobin and hematocrit numbers, allowing clinicians to estimate how much blood has been lost.

Hemoglobin, hematocrit, blood pressure, and heart rate are all very useful ways for doctors to estimate blood loss. The clinical situation is 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 the story about what’s happened to the patient, since other things can cause high heart rate and 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. As it happens, there is a severe national shortage of stored blood right now. So even though blood-letting is mostly for the history books, blood-giving is here to stay – and more needed than ever.

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 the widest possible audience. Ian earned his PhD in Greek & Latin literature and his MD at Columbia University in New York City and is now training at a hospital in Boston.