The Story of Hemoglobin / Newt InteractiveThe Story of Hemoglobin<br>How a special protein in red blood cells transports oxygen around your body<br>Jun 23, 2026

It might not seem or feel like it, but by cellular count, we are mostly blood. Over three-quarters of all the cells in our body are the 20-30 trillion red blood cells floating in arteries and veins and squeezing between capillaries to deliver oxygen wherever it’s needed. We make about 2.4 million of them every second, and they circulate tirelessly for three to four months (or 150,000 to 200,000 laps) before the spleen and liver break them down and recycle the parts.

Providing oxygen efficiently is vital to our existence, so maybe it should not come as a surprise that a significant part of us is devoted to transporting it around. But despite how crucial it is, our body doesn’t stockpile any; it has to be delivered continuously through blood. It does so with a special protein housed within red blood cells called hemoglobin, whose sole job is to pick up oxygen where it is plentiful and release it where it’s needed. Without this protein, if we relied on oxygen dissolving in blood alone, we’d need sixty to seventy times more blood circulating to stay alive.

This delivery task is harder than it sounds. When a red blood cell, chock full of hemoglobin, fills up on oxygen in the lungs and is pumped by the heart across the body, nothing tells it where to go. There is no GPS, no central coordinator steering it towards the working muscles that desperately need oxygen and away from the ones that need less. And hemoglobin itself has to keep a delicate balance: if it holds onto oxygen too tightly, or if it lets it go too easily, many tissues won’t get the oxygen they need. Getting every tissue the right amount at every moment — knowing not only how to grab and hold onto oxygen but also when to let go — requires a marvel of precision engineering. What kind of structure could possibly do this?

A brief history

For centuries, scientists have tried to figure out what blood was made of. In the 1740s, Vincenzo Menghini, an Italian physician, decided to study why blood was red. The going theory at the time was that it came from iron, the same material that forms rust, after iron’s presence was found in the ashes of incinerated vegetables several decades earlier. Exactly where this iron came from was still unknown, so Menghini decided to do an experiment: he took five ounces of blood from a dog, roasted it into a dry powder, and then hovered a naturally magnetized knife blade over it. Some bits stuck onto the blade; blood did, in fact, have iron.

He continued experimenting on other animals — an ox, a horse, a boar, birds, frogs, fishes — and found that the iron only came from the red portion of the blood after it was separated; very little came from the serum. And so, for about the next century, iron being the cause of blood’s red color remained undisputed.

But Menghini’s work raised a deeper question: why does blood carry a metal at all? Answering it meant looking inside the blood. Some seventy years before Menghini, in 1675, Antonie van Leeuwenhoek, the Dutch draper and naturalist who made the best microscopes of his time, peered at his own blood and saw tiny “sanguineous globules” floating in it. It took another century to discover the true shape of these globules. William Hewson, a British anatomist, found red blood cells not to be round but disk shaped, with a depression in the middle, like “a circular pillow that has just been punched,” as Siddhartha Mukherjee describes.

In the century that followed, attention moved to what was inside that cell. In 1840, Friedrich Hünefeld, a German biochemist, noticed that dried blood placed between two glass slides grew tiny crystals, rectangular and bright red, with sharp edges. They were the first crystals of a protein ever recorded, but he did not know what it did, and so left it unnamed. Twenty-four years later, Felix Hoppe-Seyler gave it one: hemoglobin, from hemo (blood) and globin (for its round, globe-like shape). He also showed that this protein was where the iron lived, what oxygen attached to, and what gave blood its color.

But knowing what hemoglobin was made of was not the same as knowing how it worked. How did this single molecule pick up oxygen and let it go? To understand that, you needed to see the shape. Beginning in 1937, the Austrian-born chemist Max Perutz spent more than twenty years firing X-rays at hemoglobin crystals, painstakingly reconstructing the molecule from the pattern of spots they scattered. By the end of the 1950s he had the first three-dimensional model of hemoglobin. When he compared its oxygen-loaded and oxygen-empty forms, he saw it physically change shape, clamping shut and springing open as it took on its cargo and let it go.

Thanks to Perutz and the work of hundreds of scientists before us, we can now take apart the molecule and inspect it atom by atom. Its structure reveals its...