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Blood is red, then why do veins appear blue? (Query)

Blood is red, then why do veins appear blue? (Query)

The classic question answered!

Written by Bhavyajyoti Chilukoti |Published : September 15, 2015 6:42 PM IST

My 10-year-old son asked me that blood is red in colour, but why do veins appear slightly blue in colour? Even I am confused at his question. What could be the reason for this? Please explain.

Most people have this question but don t ask. According to a study conducted by Alwin Kienle and his colleagues, published in the book Why do veins appear blue? A new look at an old question , (1) the answer for why blood is red and veins are blue is based on four major factors.

# 1:Scattering and absorption of light by the skin at different wavelengths

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Did you know light interacts with our skin at different wavelengths? When light falls on the skin surface, it penetrates the skin, is absorbed and re-emitted. The veins present underneath the skin surface re-emit small amounts of red light and high amounts of blue light, which is visible. This is one of the reasons, why veins appear blue.

# 2: Oxygenation and de-oxygenation state of the blood

Oxygen that you breathe in is carried to various parts of the body in the bound form with haemoglobin. A single molecule of haemoglobin carries four molecules of oxygen and leaves them at the target organs (where the temperature is high or the surrounding have acidic environment, etc).

When the blood carries oxygen, it is known as oxygenated blood (carried through arteries). After it is supplied to the organs and comes back to the heart, it is known as de-oxygenated blood (carried through veins). And as the oxygen levels falls (de-oxygenated blood), the colour changes from red to dark red, which in turn appears closer to blue.

#3: The diameter and depth of the veins

The second factor is the diameter and the location of the vein beneath the skin (how deep they are located). If the vein is right underneath the skin, it appears red. But if it is present deeper than millimetres, then it looks blue. This is attributed to complicated optical physics phenomena known as transport equation.

Are you aware of the fact that arteries that carry oxygenated blood appear red and the veins that transport deoxygenated blood look dark red? So, when light falls on the skin, this small difference in colour is amplified and veins tend to appear blue and arteries that have smaller diameter and located deep inside the skin look red.

#4: How the brain perceives it visually

Lastly, the visual reception process. For example, if there are red coloured and purple coloured objects, you brain perceives the purple as a shade of blue. So your brain processes whatever you visualise through the retina. So this can be one of the reasons why veins appear blue to you even though they are not.

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Reference:

  1. Kienle, A., Lilge, L., Vitkin, I. A., Patterson, M. S., Wilson, B. C., Hibst, R., & Steiner, R. (1996). Why do veins appear blue? A new look at an old question. Applied Optics, 35(7), 1151-1160.