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Photo Source: Pixabay, you shouldn't be a blue blood man, and you'd better not become a blue blood man.

In Europe, "blue blood" used to be a symbol of noble status.

In the 9th century, the Spanish aristocracy was trying to retake the Iberian peninsula from the Moors. In the description of the historian Robert Lacey, the pre-war aristocracy would raise their arms and the blue veins were particularly conspicuous on the fair skin, showing that they were pure and uncontaminated by the dark-skinned Moors.

At first, this may have been done to boost morale. The Spaniards at that time probably did not think that the concept of "aristocratic blood is not red but blue" would spread to the wider world. Because upper-class people do not often expose their skin to the sun, aristocrats are lighter than ordinary people who work outdoors all day. Blue veins are also clearer-blue blood was once popular throughout Europe to express the difference between the upper and lower layers.

We now know that human blood is not usually blue, regardless of identity or class. However, why is the vein seen from the wrist blue?

What color is the blood? High school biology tells us that arterial blood from the heart can carry oxygen to all parts of the body. After the release of oxygen, the blood will flow back to the heart along all levels of veins. As for the color of arterial or venous blood in people's eyes, it has something to do with the process of blood circulation, or the light reflected and absorbed by the blood in this process.

A beam of white light is made up of different wavelengths of light, such as red, orange, yellow, green, cyan, blue and purple. Arterial blood is red because hemoglobin, when combined with oxygen, reflects more red light into people's eyes. By contrast, hemoglobin in venous blood removes some oxygen, absorbs more red light and reflects less red light. It is difficult to maintain the bright red color of arterial blood. The dark red blood we see when drawing blood is also venous blood.

The above two tubes are venous blood (Photo: Wesalius via Wikimedia Commons, CC BY-SA 3.0)

More specifically, the part of hemoglobin that determines color is actually heme (heme). Heme is an iron-containing porphyrin compound. The ring structure centered on iron atoms contains a large number of conjugated double bonds, allowing molecules to absorb visible light and show a specific color.

When oxygen encounters hemoglobin, it binds to the iron in heme before it can be transported to other tissues or organs. A hemoglobin molecule can carry up to four oxygen molecules, and the wavelength range of light absorbed by hemoglobin varies with the number of oxygen molecules bound to it. Therefore, the color of venous blood with low oxygen content is different from that of arterial blood with high oxygen content.

But whether bright red or dark red, healthy blood is red, not blue. Where did the blue vein we see come from?

What color do you see? Putting a tube of venous blood in front of your eyes and seeing the vein through the skin are two different situations.

A beam of white light hits the skin, which is different from hitting a transparent glass container. Among the various colors that make up white light, the wavelength of red light is about 625-750 nm, while the wavelength of blue light is about 450-485 nm. In general, the longer wavelength of light is easier to penetrate the medium, red light through the skin is not a difficult task, while blue light into the skin, it is easy to be scattered in all directions, more difficult to reach the depths of the skin.

It is as if the sun emits light of various wavelengths, but the sky we see is often blue. There are many tiny particles in the atmosphere whose diameters are shorter than the wavelengths of visible light. Both red and blue light encounter these tiny particles and are scattered, but blue light has more scattering because of its shorter wavelength. This phenomenon belongs to Rayleigh scattering (Rayleigh scattering). The sky is illuminated by scattered light and becomes the blue sky in the eyes of human beings.

Photo Source: Unsplash

The situation in the skin is similar, and some blue light bounces back into people's eyes before it reaches the skin. Compared with the blue light that is easily scattered, a considerable part of the red light can reach the veins under the skin for a more complete journey.

Of course, both red and blue light can be absorbed as they pass through the skin, and both have a chance to reach the veins and be absorbed there, except that they eventually reflect as much red and blue light back into the eyes. The color of the veins we see is also related to the proportion of red and blue light returning to the eye. In fact, when white light hits the area of the vein, it usually returns more red light than blue light. Then, even if you look through the skin, most people should not see the blue veins.

Then why do we think the veins we see are blue? The vein is not painted on a piece of white paper, there is pink skin around it. The scientists found that the amount of blue light reflected in the skin area without veins below was not much different from that in the venous area, but significantly more red light was reflected in the skin area than in the venous area-it looked redder. The veins seen through the skin are not red, but may be bluish compared with the skin next to them, so they are recognized as blue by the brain.

This can be said to be an optical illusion related to color constancy (color constancy). Color constancy is a characteristic of human color perception system, which allows us to judge the color of objects under different lighting conditions. For example, the following picture simulates the effect of yellow light and blue light shining on the Rubik's cube respectively, from which people can easily find out the location of the red square.

Photo Source: Purves et al., 2016

But when these "red squares" are extracted from the original environment and put into a white background, people will feel that they are no longer red, but become orange and purple. In fact, the box did not change color, is that the result of our recognition has changed. The color of the object we recognize depends not only on the reflected light, but also on the calculation of the brain. In this way, it is not surprising that veins surrounded by ruddy skin are regarded as blue. if we put that color on a white background, we may not think it is blue.

People whose blood turns blue although under normal circumstances human blood will not be blue, but there are also some diseases that can make the blood blue. Methemoglobinemia (methemoglobinemia) is such a disease.

In the blood of healthy people, the iron in hemoglobin is almost all bivalent iron (Fe2+), which is responsible for binding oxygen. But in methemoglobin, some iron becomes trivalent iron (Fe3+), making it difficult to bind to oxygen. In this hemoglobin molecule, the affinity of the residual divalent iron ion to oxygen is enhanced, which makes it impossible to release oxygen into the tissue normally.

In general, methemoglobin accounts for less than 1% of the blood. If the methemoglobin in the blood is more than 10%, the body may develop various symptoms due to hypoxia, such as shortness of breath, cyanosis (skin or mucous membrane is bluish purple), mental state changes, headaches, fatigue, and so on. When the proportion of methemoglobin in the blood reaches about 70%, humans may be at risk of coma or even death.

Dark blue blood (photo: NEJM) among patients with this disease, perhaps the most well-known symptom is a change in blood color. When there is significantly less oxygen, the blood may appear brown or even blue. In 2019, the New England Journal of Medicine reported a 25-year-old woman with a significantly lower blood oxygen saturation, 44% methemoglobin, dark blue blood and a change in skin color.

According to the patient's own account, the night before she went to the doctor, she used a large amount of benzocaine to relieve the pain because of her toothache. This may be the cause of her sudden attack. The next morning, she felt short of breath and weak, and then went to the emergency room. After receiving intravenous methylene blue treatment, the patient's respiration was significantly improved and the skin discoloration was weakened. Eventually, the symptoms subsided completely.

Having "noble" blue blood is really not a good thing for human beings.

Reference:

Https://pubmed.ncbi.nlm.nih.gov/21085227/

Https://theconversation.com/ive-always-wondered-why-do-our-veins-look-blue-when-our-blood-is-red-83143

Https://www.historyextra.com/period/georgian/why-people-noble-called-blue-blooded/

Https://books.google.com/books/about/Aristocrats.html?id=qp-UKm46MlkC

Https://www.ncbi.nlm.nih.gov/books/NBK54103/

Https://osuwomeninphysics.wordpress.com/2015/03/11/everyday-physics-why-do-veins-look-blue/

Https://engineering.purdue.edu/~bouman/ece637/notes/ColorConstancy/color/

Https://www.ncbi.nlm.nih.gov/books/NBK537317/

Https://pubmed.ncbi.nlm.nih.gov/9588101/

Https://www.nejm.org/doi/full/10.1056/NEJMicm1816026

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by | Chestnut revision | Wang Yu

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