Is Human Blood Ever Any Color Other Than Red?

Introduction

Yes, human blood can appear green under certain conditions, particularly in deep ocean environments. To understand this phenomenon better, we need to delve into the science of color perception and the properties of light.

Understanding Color Perception

The color of an object, including blood, is not an intrinsic property. Instead, it is determined by three primary factors:

1. Incident Light: The color content of the light that illuminates the object.
2. Object Properties: How the object reflects, absorbs, and transmits colors of the incident light.
3. Detector: How the human eye or a camera detects and interprets the colors of light coming from the object.

In everyday scenarios, the incident light—such as sunlight or artificial light—contains all colors of visible light in nearly equal proportions. The healthy human eye is also capable of detecting all colors of visible light. As a result, under typical circumstances, we can consider the color of an object as primarily dependent on its own properties.

The Color of Blood

Research reported in the journal Applied Spectroscopy by Martina Meinke and colleagues provides detailed insights into the color of human blood. They measured the diffuse reflectance of blood at various conditions, including a hematocrit (the percentage of blood volume taken up by red blood cells) of $33\%$ and an oxygen saturation of $100\%$. Although the reflectance spectrum varies slightly with different hematocrit and oxygen saturation levels, the overall trend remains consistent.

In general, blood predominantly reflects red light. However, it also reflects a small amount of green light. When white light (which encompasses all colors) shines on blood, it appears red due to the much higher reflection of red light compared to green light.

The Green Color of Blood

If we illuminate blood with a light source that contains all visible colors except red, the blood will appear green. This occurs because, without red light to reflect, the blood can only reflect the green light. Therefore, the blood is literally green in this scenario, even though we typically do not notice this due to the overwhelming presence of red light reflected under normal conditions.

Blood in Deep Ocean Environments

This phenomenon is particularly noticeable in the deep ocean. Water absorbs some red light, and as we descend deeper, less red light penetrates the water. Consequently, the blood that reflects predominantly green light becomes visible when illuminated by the sunlight that reaches these depths. This can be surprising to divers who may find that a cut reveals green blood rather than the expected red.

Conclusion

In summary, while human blood is typically perceived as red, it can appear green under specific lighting conditions, such as in the deep ocean or when illuminated by light sources devoid of red. This is not a transformation of the blood itself but rather a result of the complex interaction between light and the properties of blood. Thus, we can conclude that human blood is always somewhat green; it just becomes apparent under the right conditions.