A diver was bitten by a moray eel underwater and his blood turned green...

Tim Powell is a diving enthusiast. In 2010, he uploaded a video showing that his finger was accidentally bitten by a small moray eel and was bleeding in about 20 meters of the ocean.

At this time, his blood was neither bright red nor dark red, but green, like emerald.

The diver bleeds green. Source: YouTube

In fact, it is not just humans, fish that are wounded in the deep sea also bleed green. Sometimes, divers use spear guns to catch fish underwater. If they reach a certain depth, the wounds of the fish will ooze green blood, but their blood is obviously red when they are killed on the shore.

Image source: YouTube

Image source: YouTube

Of course, it doesn't necessarily need to be on shore. Even in shallow sea areas, the blood of cut fish will also be red.

Image source: YouTube

It should be noted that the blood of most fish is red. This is because their blood contains hemoglobin, a protein rich in iron that can bind to oxygen and transport it around the body. Hemoglobin appears red when it binds to oxygen, which is why the blood of most animals, including humans and fish, appears red on land.

Image source: Internet

The color of blood, whether on the surface or under deep sea pressure, is basically determined by the state of iron atoms in hemoglobin combined with oxygen. The color of blood changes mainly from bright red (oxygen-rich blood) to dark red (oxygen-poor blood).

But we have to know that no matter what color it is, we see it with our eyes. High school physics class tells us that any object we see is because light is reflected into our eyes. The reason why red blood appears red is because it absorbs other spectra and reflects red light.

Image source: Internet

The phenomenon of red blood appearing green in the deep sea is not due to a change in the color of the blood itself, but rather to the specific underwater lighting conditions and the way human vision is perceived.

There are several main factors involved here: water's light absorption characteristics, the human eye's color perception mechanism, and the color reflection of blood.

Water absorbs light of different wavelengths, but this absorption has different effects on wavelengths of light of different colors. In water, the absorption of light is mainly caused by the water itself and the substances dissolved in it (such as organic matter and minerals) and suspended particles.

When sunlight enters water, red light, due to its longer wavelength, has lower energy and is more easily absorbed by molecules and other substances in the water and converted into heat energy, rather than continuing to propagate forward. This means that below a certain depth, most red light has been absorbed by water, while short-wavelength light such as blue and green light can still penetrate.

Image source: Internet

Human vision perceives color through cone cells on the retina in the eye, which are sensitive to different wavelengths of light. Under normal lighting, the human eye can perceive a variety of colors. However, in the deep sea environment where only blue light can reach, there is a lack of sufficient red light, which affects our normal perception of color. Red is difficult to be observed normally in the deep sea.

In addition, we also need to know the diffuse reflectance of human blood. Scientists have found that under natural light, human blood not only reflects red light (mostly), but also slightly reflects some green light , as shown in the figure below.

Diffuse reflectance of human blood, hematocrit 33%, oxygen saturation 100% Image credit: Christopher S. Baird

So, in the deep ocean, in the absence of red light sources, blood appears green.

This is why some smart watches use green LEDs to monitor heart rate. Green light is more sensitive when it comes to detecting changes in blood flow. When green light passes through the skin and blood vessels, small changes in the amount of blood will cause changes in the intensity of the reflected or transmitted light, and heart rate can be calculated by analyzing these changes.

Image source: junkie

If a diver in the deep sea does not rely on natural light, but uses his own artificial lighting, such as a flashlight, to illuminate his injured hand, he will find that his blood will turn red again.

This is why most deep-sea animals are red. For example, the newly discovered single-spined handfish is a lovely red.

Image source: SCHMIDT OCEAN INSTITUTE

In the deep ocean, red animals can appear almost black or completely transparent because red light is almost completely absorbed. This color makes them nearly invisible to predators, providing an effective means of camouflage. The red color becomes extremely difficult to detect in the deep ocean, allowing these creatures to better avoid the attention of predators.

(You can see red in the picture because of artificial light, natural light is another state)

Red whale image source: Kunio Amaoka

In addition, organisms in deep-sea environments need to survive in extreme conditions, including low temperatures and high pressures. Although the direct effect of red on heat preservation or energy absorption may be limited, this color may be related to some special biological compounds in their bodies that are used to absorb and utilize the very small amount of light energy in the surrounding environment.

If you see green blood in a game, it has nothing to do with science...

References

[1]https://pubmed.ncbi.nlm.nih.gov/12819275/

[2]https://www.nationalgeographic.com/animals/article/walking-fish-deep-sea-new-species-chile

[3]https://opg.optica.org/as/abstract.cfm?uri=as-59-6-826

Planning and production

Source｜Bring Science Home (id: steamforkids)

Author: Su Chengyu

Editor: Yang Yaping

Proofread by Xu Lai and Lin Lin