Blood Contains Iron, So Why Doesn't It Stick to a Magnet?
― Iron atoms are locked away one by one, and blood actually pushes very slightly against a magnet
"Blood is red because it contains iron." That's true. But if you think, "so a strong magnet should pull blood toward it," you've got the wrong idea. Iron sticks to a magnet not because of the "element" itself. It's a property of how the atoms are "arranged": packed tightly together, all lined up in the same direction.
You cut your fingertip on a piece of paper, and a small bead of blood wells up. The blood tastes a bit like iron.
Come to think of it, you learned at school that blood contains iron. If you brought a strong fridge magnet close, would the bead of blood be pulled toward it?
You try it, and nothing happens. But if iron is flowing all through your body, wouldn't your blood be tugged around when you go inside the big magnet machine at a hospital?
There are just two reasons it doesn't stick
In an iron nail, the atoms are packed tightly, touching one another, and they line up their tiny magnets all at once. In blood, the iron is locked away one atom at a time inside a large protein. With no neighbours to link up with, the atoms can't line up.
Water has a property that makes it push very slightly against a magnet. The tiny pull on the iron in blood loses out to this push. Overall, blood is on the side that backs away, just a little, from a magnet.
Both reasons become clear once you drop the assumption that "iron always sticks to a magnet." Let's take them in turn.
Iron sticks to a magnet because its atoms form a "team"
Each atom can be a tiny magnet, too small to see. Iron atoms are no exception. But a single atom's magnet is very weak, and its direction is easily scrambled.
Inside a nail or an iron plate, the iron atoms are packed with no gaps. Neighbouring atoms exert a strong force on each other that makes them point the same way. As a result, "zones" form in which trillions of atoms all point in the same direction. When you bring a magnet close, these zones swing into line together. That is why a nail is pulled strongly toward it.
The iron in blood sits inside a large protein called haemoglobin. Each haemoglobin molecule holds just four iron atoms, and those four are far apart from one another. They can't line up with a neighbouring iron, so no "team" can form. Compare the left and right sides of Figure 1.
The body holds roughly 3 to 4 grams of iron in an adult, and about two-thirds of it is in the haemoglobin in blood. By amount, that is about one small nail. Even this much iron would stick to a magnet if it were solidified into the shape of a nail. The decisive difference is that in the body it is dissolved in 5 litres of blood, split into single atoms.
Water pushes slightly against a magnet
The other reason is that most of blood is water. Water doesn't just fail to stick to a magnet. Bring a strong magnet close and it is pushed back very slightly. This is true of most materials, not only water. It happens because the electrons inside atoms shift their motion in a way that opposes the magnet's force.
So is the iron in blood not pulled at all? In fact, haemoglobin that has let go of its oxygen (more common in blood in the veins) is pulled very slightly toward a magnet. While it is holding oxygen, it isn't pulled. Even so, the push from the water is far bigger. So whether it is arterial or venous, blood as a whole falls on the side that backs away, just a little, from a magnet.
Try Figure 2 to see how weak this is. When you change the magnet's strength with the slider, you can compare the "weight" on one litre of blood with the size of the "force the magnet pushes back with."
Even with a strong magnet like those used on fridges, the push is only about 1% of the blood's weight. That is why the bead of blood on your fingertip doesn't so much as twitch. To float blood, you would need an apparatus about 90 times stronger. In an experiment with just such an apparatus, a living frog, which contains a lot of water, floated in mid-air (Is it true that water repels magnets?).
Blood carrying oxygen and blood that has let it go react very slightly differently to a magnet. The first to measure this were the chemist Linus Pauling and his colleagues, in 1936. About 50 years later, Seiji Ogawa and colleagues in Japan took note of this difference. They showed that MRI (the big magnet machine at a hospital) can reveal where in the brain oxygen is being used. Today it is a standard way to find out "which part of the brain is working."
An MRI's strong magnet does not tug on the blood in your body. The danger is small iron objects whose atoms are packed tightly together, such as keys or hairpins in a pocket. These can fly toward the machine with great force. That is why you are asked to remove metal before a scan.
Summary
Blood certainly contains iron. But that iron is locked away inside haemoglobin, one atom at a time and far apart. The atoms can't link up and line their directions, so blood doesn't stick to a magnet the way a nail does. On top of that, most of blood is water, which pushes very slightly against a magnet. So blood as a whole falls on the side that backs away, just a little, from a magnet.
What decides whether something sticks to a magnet is
less whether iron is there than how the iron is arranged.
If you want to know more about water being pushed away by magnets, read Is it true that water repels magnets? too. How atoms line up inside a magnet is covered in detail in If you cut a magnet in half, do you get a magnet with only a north pole?
- Skewer one grape on each end of a drinking straw, then hang the middle from a thread, or rest it on the base of an upturned cup, so it balances level.
- In a spot out of the wind, wait until the grapes stop moving completely.
- Gently bring a strong neodymium magnet close to one grape, stopping short of touching it. The grape slowly begins to turn, as if running away from the magnet.
A grape is mostly water, so like blood it pushes very slightly against a magnet. The movement is very slow, so be patient. Neodymium magnets attract each other hard enough to pinch your fingers, so don't bring two close together. Keep them out of reach of small children so they don't swallow them.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school physics, chemistry and biology
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university courses (electromagnetism, condensed matter physics, biophysical chemistry)
- ResearchNot taught even at university as settled fact; questions researchers are studying right now
Middle schoolTerms: this phenomenon has names
- Ferromagnetism: the property, seen in metals such as iron, nickel and cobalt, in which the atoms' tiny magnets line up with their neighbours and are strongly pulled by a magnet.
- Paramagnetism: the property in which each atom is a tiny magnet, but with no neighbours to line up with, it is pulled only very slightly by a magnet. Haemoglobin that has let go of its oxygen (deoxyhaemoglobin) is an example.
- Diamagnetism: the property of being pushed very slightly away by a magnet. Water, grapes and most of the human body are examples.
- Haemoglobin: a protein inside red blood cells that carries oxygen. Each molecule has four iron atoms, and these irons grab the oxygen.
Middle schoolHigh schoolChecking with formulas: how hard does a magnet push blood back?
First we estimate how much iron there is in one litre of blood. Then we compare the force with which a strong magnet pushes blood back against the weight of the blood.
| In symbols | f ＝ ( |χ| ÷ μ0 ) × B × ( dB/dz ) , floats when f ＝ ρ × g |
| In words | Push per unit volume ＝ size of magnetic susceptibility ÷ permeability of vacuum × field strength × field gradient. If this reaches "density × gravitational acceleration" (the weight per unit volume), the material floats |
| Where it comes from | An object in a magnet's field lowers its energy by moving toward stronger field (or, if diamagnetic, toward weaker field). The derivative of that energy with position is the force. If the field is uniform, the gradient is 0, so no force acts however strong it is |
| Meaning of symbols and units | χ: magnetic susceptibility (no unit), μ0: permeability of vacuum (tesla × metre ÷ ampere), B: field strength (tesla), dB/dz: field gradient (tesla per metre), ρ: density (kilograms per cubic metre), g: gravitational acceleration (metres per second squared) |
| Size of blood's magnetic susceptibility (about the same as water's) | 0.000009 |
| Permeability of vacuum | 0.00000126 |
| Field near the surface of a strong neodymium magnet (rough value) | 0.4 tesla |
| Field gradient at the same place (rough value, assuming a drop of 0.4 tesla over 1 cm) | 40 tesla per metre |
| Density of blood | 1060 kilograms per cubic metre |
| Mass of one haemoglobin (per mole) | about 64500 grams |
| Mass of one mole of iron | 55.8 grams |
| Haemoglobin in one litre of blood (rough value) | 150 grams |
| Iron in one mole of haemoglobin | 4 × 55.8 ＝ 223.2 grams |
| Share of iron in the haemoglobin's mass | 223.2 ÷ 64500 ≒ 0.00346 |
| Iron in one litre of blood | 150 × 0.00346 ≒ 0.52 grams |
| Iron in all the blood in the body (5 litres) | 0.52 × 5 ＝ 2.6 grams |
| Field × field gradient | 0.4 × 40 ＝ 16 |
| Multiply by the size of the susceptibility | 16 × 0.000009 ＝ 0.000144 |
| Divide by the permeability of vacuum (push, in newtons per cubic metre) | 0.000144 ÷ 0.00000126 ≒ 114 |
| Weight of one cubic metre of blood (newtons) | 1060 × 9.8 ＝ 10388 |
| Push ÷ weight | 114 ÷ 10388 ≒ 0.011 |
| "Field × gradient" needed to float it (step 1) | 10388 × 0.00000126 ≒ 0.01309 |
| "Field × gradient" needed to float it (step 2) | 0.01309 ÷ 0.000009 ≒ 1454 |
| How many times a fridge magnet | 1454 ÷ 16 ≒ 91 |
One litre of blood holds about 0.5 grams of iron, and the whole body has about one small nail's worth. Even with a strong neodymium magnet, the push stays at about 1.1% of the blood's weight. To float blood, you would need about 90 times as much "field × gradient." That strength is only just reached by special electromagnets in research labs.
High schoolHigh school+Electron "spin" and whether they pair up
High schoolThe source of an atom's tiny magnet is the electron. Each electron is a tiny magnet. Normally electrons form pairs with opposite directions, and cancel each other's magnetism. If an electron is left over without a partner, the atom becomes a tiny magnet.
High school+The iron in haemoglobin is a divalent iron ion. In the state where it has let go of oxygen, it has four unpaired electrons. That is why it shows paramagnetism. When it grabs oxygen, the arrangement of the electrons changes, and the magnets of the leftover electrons can no longer be seen from outside. Pauling and Coryell showed this difference by magnetic measurements in 1936.
UniversityExchange interaction and magnetic susceptibility
Ferromagnetism arises because an exchange interaction acts between the electrons of neighbouring atoms, so that lining up their directions lowers the energy. The result is regions with aligned directions, called magnetic domains. The irons in haemoglobin are too far apart for the exchange interaction to act at all. So they behave as a collection of independent magnets and show paramagnetism that follows Curie's law. In terms of volume magnetic susceptibility, venous blood is only slightly less diamagnetic than arterial blood. The phenomenon in which this difference shows up in the MRI signal is called the BOLD effect (blood-oxygen-level-dependent signal), and it is the basis of functional MRI.
📖 Derivations of the formulas and further reading: Paramagnetism (Wikipedia, in Japanese) ／ Ferromagnetism (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- The link between brain signals and neural activity. What functional MRI shows is changes in blood oxygen. How accurately that corresponds to neural activity is still being studied.
- Living things' "magnetic sensors." How migrating birds and other animals sense the Earth's magnetic field is unsettled. One idea points to molecular reactions in the eye, another to magnetic particles in the body.
- Blood flow in strong magnetic fields. In a very strong field, it is known that electricity arises in flowing blood and changes the flow slightly. How far stronger MRI machines may affect the body is being investigated.
In other words, the content of this article, too, is an "explanation within what is understood today." Still, the conclusion itself, that "blood does not stick to a magnet," has been confirmed by decades of measurements.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Magnetic fields", "Functions of blood" | What sticks to a magnet and what doesn't; red blood cells carrying oxygen |
| High school | Physics: "Magnetic field", Chemistry: "Electron configuration", Biology: "Oxygen transport" | Electron pairs and magnetism; the iron in haemoglobin |
| High school+ | Advanced chemistry: "Complexes" | How iron's electron arrangement changes with or without oxygen |
| University | Electromagnetism, condensed matter physics, biophysical chemistry | Exchange interaction, magnetic domains, Curie's law, BOLD effect |
| Research | Brain science, magnetic sense in living things | Interpreting functional MRI; migrating birds' magnetic sensors |
| ― | Links to daily life | Why metal is removed before an MRI scan; scans that examine how the brain works |
- Pauling, L. & Coryell, C. D. (1936) The Magnetic Properties and Structure of Hemoglobin, Oxyhemoglobin and Carbonmonoxyhemoglobin. PNAS 22, 210–216.
- Ogawa, S., Lee, T. M., Kay, A. R. & Tank, D. W. (1990) Brain magnetic resonance imaging with contrast dependent on blood oxygenation. PNAS 87, 9868–9872.
- Berry, M. V. & Geim, A. K. (1997) Of flying frogs and levitrons. European Journal of Physics 18, 307–313.
- Chikazumi, S. (近角聰信), Physics of Ferromagnetism, Vol. 1 (強磁性体の物理（上）), Shokabo (裳華房)
- Paramagnetism (常磁性) - Wikipedia (in Japanese)
※This article is a science explainer for general readers. The figures given are rough estimates meant to help you understand how things work. The amount of iron in the body and the makeup of blood vary from person to person. For MRI scans, follow the instructions of your medical institution.