Why is blood red?

Blood is red because it contains red blood cells which are red (surprisingly enough). Blood is 55% liquid and 45% cells. Of these cells, about 98% are red – more than enough to give our life-giving fluid its rich red colour. Quite why red blood cells are red requires a more detailed answer. But when you’ve red (sorry) what follows, you will certainly know why…

What are red blood cells anyway?

Red blood cells are the little chaps responsible for the transport of oxygen around the body. They perform this important job thanks to a protein called haemoglobin; a substance that they are stuffed to the gills with. In fact, they are so crammed with haemoglobin that red blood cells don’t even contain a cell nucleus (something pretty much all other cells have). Haemoglobin contains iron and is red in colour. Zooming in even further reveals more.

Red like rust

Figure 1: Haemoglobin
Figure 1: Haemoglobin

If you had a microscope good enough to see haemoglobin up close, the first thing you would notice is that it is actually four separate proteins stuck together (figure 1). Each of the protein structures contains a special region (called an ‘active site’) known as a heme group. Zoom in closer onto one of these heme groups and you would see that each is also made up of four structures (called pyrrole molecules) linked together in a ring to form what is called a porphyrin (see figure 2). At the centre of this porphyrin rests a chunky atom of iron atom, held in place by four nitrogen atoms (figure 2). This iron atom is responsible for haemoglobin’s oxygen carrying powers and when iron bonds with oxygen it turns red, in a similar way to the iron tools react with the oxygen in the air to form red rust on their surface.

To the core of haemoglobin’s oxygen infatuation

Figure 2: Heme molecule. Each letter represents an atom; the lines represent bonds.
Figure 2: Heme molecule. Each letter represents an atom; the lines represent bonds.

The story could stop there but there are even more secrets to haemoglobin. There is a fifth nitrogen atom that holds the iron in place but you might not notice it at first because it sticks out at a right angle to the porphyrin ring (figure 3). This fifth nitrogen links the iron atom to a ‘histidine’ molecule that is connected to a different part of the haemoglobin. Oxygen molecules can bond with the iron atom at the centre the whole arrangement, but when this happens, there is a structural change: the entire heme protein morphs into a shape. And as it does this, it makes it easier for subsequent oxygen molecules to attach to the other heme groups.

The nifty trick helps speed up the loading of haemoglobin with oxygen as it moves through the body. It happens because the iron atom is actually too large to fit inside the porphyrin, meaning that it sits slightly out of the porphyrin plane. The size of the iron diminishes when it binds with the oxygen, allowing it to slide into the porphyrin plane. And in doing so, it tugs on that histidine group, triggering a sequence of structural changes that ripple throughout the entire protein. Once all four heme groups are oxygenated (attached to oxygen molecules) we refer to the protein as oxyhaemoglobin. In the body, the blood is oxygenated when it passes through the lungs. (The non-oxygenated form of haemoglobin is incidentally called deoxyhemoglobin.)

Colouring in the gaps

Figure 3.
Figure 3.

Maybe you have heard the saying that blood without oxygen is blue? Well, anyone who has had blood taken from a vein by syringe will know that this is simply not true. Blood from the veins is definitely a much darker red than the oxygenated blood in the arteries, but it is a long way from blue.

Scientists at the University of Groningen in The Netherlands wanted to probe the reasons for these colour differences. They measured how much light oxyhemoglobin and deoxyhaemoglobin absorbed at different wavelengths (figure 4). They found that oxyhemoglobin strongly absorbed wavelengths of 540-580nm and 480-500nm, corresponding to green/yellow and blue light respectively. Most importantly, oxyhaemoglobin absorbed very little light above 620nm (corresponding to red), instead reflecting this toward our eyes.

If we now look at the curve for deoxyhaemoglobin (blue line figure 4), we see that the molecule absorbs mostly green light at 560nm and, importantly, absorbs less blue light at 480nm and more red light at 620nm. This means deoxyhaemoglobin reflects more blue and less red light, giving it a darker appearance than oxyhaemoglobin. These differences probably explain the blue/purple colour in the skin of patients suffering from low blood oxygen levels.

How to make your blood turn green

Figure 4.
Figure 4

Knowing that haemoglobin exists in two forms and that the major difference between them is the presence, or lack of, an oxygen-iron bond, we can theorise that it is the iron that is the main contributor for the colour of blood. With that in mind, what might happen if we start messing about with the state of the iron or swap the iron with a different metal all together?

Sulfhaemoglobinemia is a condition caused by incorporation of a sulphur atom into haemoglobin, which permanently binds to the iron. This unpleasant condition has the effect of turning human blood green! In one case from 2007, a patient had blood drained from his leg and surprised his doctor when he bled green blood instead of the more traditional red. It was later found that the patient had been taking large doses of a sulphur-containing drug called sumatriptan to treat his migraines. When he stopped taking the medication his blood soon returned to a more normal red colour.

Members of the royal family don’t have blue blood, but some creatures do. The horseshoe crab, for example, doesn’t use haemoglobin at all for transporting oxygen around the body. Instead the crab uses haemocyanin, a molecule which uses two copper atoms to bind an oxygen molecule. Binding oxygen in this way causes the copper to turn bright blue. And interestingly, horseshoe crab blood is widely used in the pharmaceutical industry as a non-toxic way of sterilising vaccines and injectable medications. Not to be tried with a monarch’s blood.

Answer by Thomas Donoclift

Photo credits:

Figure 1 & 2: http://en.wikipedia.org/wiki/Hemoglobin – Creative Commons Licence
Figure 3: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/d-Block_Elements/Transition_Metals_in_Biology – Creative Commons Licence
Figure 4: Modified from data originally presented in reference 1 but can be found on http://quizlet.com/7051703/bc-ch-7-hemoglobin-and-myoglobin-flash-cards/
Front slider image: Blood drop by Fredrik Rødland, on Flickr


Spectrophotometry of Hemoglobin: AbsorptionSpectra of Bovine Oxyhemoglobin, Deoxyhemoglobin, Carboxyhemoglobin, and Methemoglobin
W. G. Zijlstra and A. Buursma Department of Pediatrics and Department of Medical Physiology,
University of Groningen, Groningen, The Netherlands
Metal Complexes in the Blood for Oxygen Transport Inorganic Synthesis Experiment
Rachel Casiday and Regina Frey
Department of Chemistry, Washington University St. Louis

Article by Thomas Donoclift

March 27, 2015

Thomas Donoclift is currently working towards his PhD in radiation chemistry at the University of Manchester, UK. He is based in sunny West Cumbria; close to the heart of Britain's nuclear industry, at the University’s Dalton Cumbrian Facility. Thomas enjoys music, board games, and subjecting aqueous solutions to harsh radiation fields. He's nice really.

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