1. How Doctors in Europe Used Bloodletting Long Ago
Long ago, doctors in Europe would cut a vein in a patient’s arm to let blood flow out if they had a fever or felt sick. This practice was called bloodletting [1]. At that time, people believed that the body contained four types of fluids. They thought that illness happened when these fluids were out of balance [1]. If doctors believed there was too much blood, they thought removing it would restore balance and cure the patient [1].
George Washington, the first president of the United States, received this treatment the day before he died [2]. He was suffering from a painful throat condition. After having about 40% of his body’s blood removed, he passed away [2]. Some medical historians believe that bloodletting alone likely did not kill a man of Washington’s size. They suggest that the real danger was the throat infection itself. At that time, doctors rarely performed tracheotomies, which are surgeries to help breathing [2].
2. Why Blood Is Thicker and Harder to Flow Than Water
Water flows easily. However, blood is much thicker and harder to flow because it contains many tiny particles called red blood cells [3]. In science, this resistance to flow is called viscosity [3]. Imagine a swimming pool. If only a few people are in the water, it is easy to swim. But if the pool is packed with people, it becomes difficult to move through the water. Blood works in a similar way. The more red blood cells there are, the thicker the blood becomes, and the harder it is to flow [3]. Additionally, when body temperature drops, blood becomes even thicker [3].
3. Polycythemia Vera: A Disease That Makes Blood Too Thick
There is a condition called polycythemia vera, where the blood becomes too thick [4]. In this disease, a problem occurs in the bone marrow, which is where blood is made. The body produces far too many red blood cells [4]. When there are too many red blood cells, the blood’s viscosity increases, making it hard to flow. This can cause headaches, dizziness, and ringing in the ears [4]. Because the blood is moving slowly, clots (hard masses of blood) can form more easily. These clots can lead to serious conditions such as strokes or heart attacks [4].
Surprisingly, the main treatment for this disease is bloodletting [4]. Doctors remove 400 to 600 milliliters of blood at a time. This lowers the percentage of red blood cells to a level closer to that of a healthy person [4]. In the past, bloodletting was based on the incorrect idea of balancing body fluids. But today, it is used to lower blood thickness and prevent clots. This is a proven reason. The same action—removing blood—has a completely different reason now. Removing blood does not help with ordinary fevers or feeling unwell. It is a special treatment used only when a doctor confirms that the blood is too thick [4].
Note: When blood flows poorly, clots can form, which may lead to dangerous conditions like strokes or heart attacks [4].
4. How Sickle Cell Disease Changes the Shape of Red Blood Cells
It is not just the number of red blood cells that affects blood thickness. The shape of the cells matters too. A condition called sickle cell disease shows how changing shape can block blood flow [5]. Normally, red blood cells are disc-shaped. But in this disease, when oxygen levels in the body drop, the cells change shape. They become long and thin, like a sickle (a curved blade) [5]. Imagine a soft, round ball turning into a hard crescent moon shape. These changed cells are fragile and tend to stick to the walls of blood vessels [5]. They can block narrow blood vessels, stopping the flow of blood [5]. The area where blood stops has even less oxygen, causing more cells to change shape. This cycle can cause severe pain in the body [5].
5. Why Blood Flows Better Through Very Narrow Blood Vessels
You might think that as blood vessels get narrower, red blood cells would bump into each other more, making the blood harder to flow. However, the opposite happens [6]. In 1931, two scientists conducted an experiment using glass tubes of different widths. They found that in tubes as narrow as capillaries (the smallest blood vessels), the viscosity of blood actually decreased as the tube got narrower [6]. Why does this happen? Inside these narrow tubes, red blood cells move to the center. Near the walls, a thin layer of plasma (the liquid part of blood) forms, which contains almost no red blood cells [6]. Think of people walking in a narrow hallway. If they walk in a single line down the middle, they can move more smoothly than if they try to spread out sideways. Something similar happens in narrow blood vessels [6].
It is said that if you connected all the blood vessels in one adult’s body end-to-end, they would stretch about 100,000 kilometers (about 62,000 miles). Most of this length is made up of capillaries [7]. The blood can keep flowing through this long, narrow path without stopping thanks to this change in viscosity [7].
Note: The total length of blood vessels in an adult is estimated at around 100,000 kilometers, with capillaries making up most of it [7].
6. What Happens to Your Body After Blood Donation
Have you ever donated blood, or heard about a family member doing it? After blood donation, the body gradually replaces the lost components. The water in the blood returns quickly, while the red blood cells take a few weeks to replace themselves [8]. If you have donated blood, try to remember what happened. You might ask your family how much blood was taken or what you drank afterward. It is an interesting way to think about how your body works.