Blood is obviously red, so why do blood vessels appear blue-green?

Recalling the process of intravenous drip, I believe that everyone has some lingering fears. Especially for those who have thick flesh on the back of their hands and cannot see the blood vessels clearly, the nurse in charge of the injection will use a rubber band to tie the wrist, forcing the blood vessels to bulge on the back of the hand, and then pinch the needle and slowly insert it into the blood vessel.

At this point, the unspeakable feeling of oppression will surely make many people turn their heads away, while those with strong psychological qualities will watch the whole process. As for some cruel and heartless people, they can stare at the nurses so hard that their hands tremble. When the needle is inserted and the blood flows into the catheter due to blood pressure, they may even stare at the dark red blood and wonder curiously: My blood is obviously red, but why are these blood vessels blue-green?

Source: aijiutop.com

1

The blue color of blood vessels is related to light

Our blood vessels are located deeper beneath the skin. When light penetrates our epidermis, Rayleigh scattering occurs. The shorter wavelength blue light will be reflected in large quantities, while the longer wavelength red light can penetrate deeper into the blood vessels until it is absorbed. As the red light is absorbed, the reflection of the blue light becomes more obvious.

Of course, if the blood vessels are distributed under the shallow epidermis (about 0.5mm depth), and then exposed to sunlight, the blood vessels will faithfully reflect their original bright red color, such as the blood vessels on the rabbit's ears.

As we all know, light is an electromagnetic wave with different wavelengths and frequencies. Light waves within the frequency range that the human eye can perceive are called visible light. The human body's cone cells and brain nerves can process visible light of different frequencies into different colors , which is why we can recognize colors.

Source: Wikipedia

If we decompose sunlight, we can see a rainbow. Based on the color distribution of the rainbow, we can simply see the law of visible light frequency and wavelength. The closer to red light, the longer the wavelength of light and the lower the frequency. The closer to purple light, the shorter the wavelength of light and the higher the frequency.

Source: Wikipedia

When light propagates in an inhomogeneous medium (such as the atmosphere), it will scatter. If the diameter of these medium particles is smaller than the wavelength of light, the light will undergo Rayleigh scattering - the particles will scatter the light in all directions, and its reflection intensity is inversely proportional to the fourth power of the wavelength. In other words, compared with red light with a longer wavelength, blue light with a shorter wavelength will undergo stronger Rayleigh scattering. The sky is blue because atmospheric molecules scatter a lot of blue light, and it is not purple because the human eye is not sensitive to purple light.

So what does this have to do with blood vessels?

2

Color constancy

The above introduces the influence of reflected light on the color of an object, but the specific color of an object is actually greatly affected by the color processing mechanism of the human brain. Friends who often come into contact with photography must know that the camera has a function called white balance , which can balance the white in the photo in combination with the color temperature value we set for the camera to restore the original color of the object.

The human body's color processing mechanism also has this function, with higher precision and accuracy. When normal white light shines on an object, the color of the object is called the intrinsic color . If a light source of other colors shines on the object, our brain can still quickly distinguish the intrinsic color of the object. Our judgment of the color of the object is basically not affected by the color of the illuminating light source . This magical phenomenon is called color constancy .

Just like the strawberry in the picture below, which is illuminated by blue-green light, if you cover the other parts, you will find that the strawberry will appear gray in this case, but if you combine it with the overall environment, you can immediately judge that the strawberry is originally red. Color constancy is an ability that humans have acquired after hundreds of thousands of years of evolution. It can help us quickly identify the current environment and avoid being confused by the surface color of objects under special lighting conditions, greatly improving human survival ability.

We can still tell that the strawberry is red when it is illuminated by blue-green light.

Source: Zhihu

But this ability is not perfect. When the lighting conditions are too extreme or the lighting information of the surrounding environment is missing, it is easy for us to fall into misunderstandings. The most typical example is the yellow and white/blue and black dresses that caused countless people to argue on the Internet a few years ago.

Source: Internet

What I can tell you is that according to the explanation provided by the person who provided the photo, the inherent color of this dress under normal white light conditions is blue-black. However, in the photo, the strong yellow light directly makes the dress lose its original color. At the same time, because the dress occupies the entire photo, the lighting conditions of the surrounding environment are missing, and the color constancy mechanism lacks the lighting reference of the overall environment , which leads to countless people making misjudgments. This can be called a large-scale scene of the collapse of the worldview.

The color constancy mechanism lets us know that the color of an object is also related to the lighting conditions of its surrounding environment. Back to the blue blood vessels, the dense blood vessels under the skin are inseparable from the surrounding tissues. The light reflected by the surrounding tissues will greatly interfere with the human body's color constancy mechanism, so the blood vessels will appear blue-green in our eyes.

The ancients said that what you hear is false, what you see is true. But now with the development of science and technology and society, this saying seems less and less reliable. Today, after understanding the visual mechanism of some parts of the human body, we know that even if we just stare at a silent photo, even if we only see an ordinary skirt, we will be deceived by our eyes. Are you so sad that you have no tears to cry?