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Why do veins look blue?

Blood is red, but thanks to physics, chemistry and industrial relations, veins can look blue, explains Dr Karl.

Back in the olden days, people of nobility were called 'blue bloods'. You see, the nobility had lives of leisure, and certainly did not work in the fields. And as a result, you could easily see the blue veins just under the surface of their untanned skin.

On the other hand, the peasants would've had darker, tanned skin and it was harder to see the blue blood in their veins.

But why do the nobility, the peasantry, anybody, have veins that appear blue? After all, when you accidentally cut yourself you see that human blood is red not blue.

Your blood, all five-or-so litres of it, is pumped around your body by your heart. The blood leaves your heart at high pressure and is a light cherry-red colour because it's loaded with oxygen. It travels through your arteries and around your body. Oxygen is extracted and used by body tissues such as the brain, an exercising muscle, or even your skin.

In the tissue, the blood dumps its load of oxygen while simultaneously picking up carbon dioxide. As blood loses its oxygen, it changes in colour slightly from light cherry-red to dark red. It then flows from the smaller veins into the bigger veins, gets back to the heart, and gets pumped into the lungs, where it regains its oxygen and light cherry-red colour.

But at no stage does the blood turn a bluish colour. It's always a shade of red. So, why are veins blue?

The truth is that they aren't blue at all. Surgeons can tell you that without the overlying skin, a vein carrying blood is not blue. It's blue only when you look at it through the skin.

Why? The best answer comes from a paper in the field of optical physics, written back in 1996. It was called "Why do veins appear blue: A new look at an old question" and was written by Alvin Kienle and colleagues.

The blue colour of veins turns out to be caused by four separate factors.

The first one is how light interacts with the skin at different wavelengths or colours. Light does a lot of stuff. It'll penetrate the skin, it'll be absorbed, and finally it'll be re-emitted. This process of light being absorbed and then re-emitted happens many millions of times in the blink of an eye, as the light goes into, and then out of, the skin.

The scientists found that the veins near the surface re-emitted tiny amounts of red light, but lots of blue light. This means the colour blue is more noticeable.

The second factor was how much oxygen the blood was carrying. Most of the oxygen in blood is carried by very large molecules called haemoglobin. Fully loaded, haemoglobin can carry four molecules of oxygen. But, if the conditions are right, such as high temperature or acid environment and so on, one or more of the oxygen atoms will leave the haemoglobin. As the oxygen level of the haemoglobin goes down the colour changes from light red to a darker red, which is still red but getting closer to our mysterious blue.

The third factor is the veins themselves, specifically what is their diameter and how deep beneath the skin they are. If the vein is right underneath the skin it will appear reddish. But the overwhelming majority of veins are deeper than half a millimetre. In this case, thanks to the complicated optical physics involved in the so-called 'transport equation' the vein will look more blue.

Now you remember that deoxygenated haemoglobin in veins was a darker shade of red than oxygenated haemoglobin in arteries. This rather small difference is amplified as the light travels through the skin, and the overall result is that in comparing arteries and veins, the veins will look more blue. And, because the arteries are mostly smaller in diameter and deeper down they will usually not be seen at all.

Finally, there's the fourth factor — your brain. Your brain does a lot of processing of what comes in from the retina. For example, is the colour purple always purple? No. If a purple object is next to the colour red your brain will turn the purple into a shade of blue.

In the case of veins under the skin, the contrast of the surrounding skin will also tend to make the veins a blue colour. (Check out this optical illusion as an example — the 'green' and 'blue' spirals are actually the same colour.)

And with all this talk of blood, I hope you listeners aren't turning a whiter shade of pale ...

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