Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Is human blood ever any color other than red?

Category: Biology      Published: July 16, 2015

Yes, human blood is green in the deep ocean. We have to be careful about what we mean by color. Objects don't really have an intrinsic color. Rather, the color of an object is determined by three factors: 1) the color content of the incident light that is illuminating the object; 2) the way the object reflects, absorbs, and transmits the incident colors of light; and 3) the way in which the detector such as your eye or a camera detects and interprets the colors of light coming from the object. In everyday life, the incident light (such as from the sun or from a light bulb) typically contains all colors of visible light in nearly equal proportions. Furthermore, the healthy human eye can detect all colors of visible light. For these two reasons, in typical circumstances, we can treat the color of an object as only depending on the properties of the object itself. However, once we move away from typical circumstances, we have to use the more complete description of color, which involves the light source, the object, and the detector. With this in mind, let's turn to the color of blood.

As reported in the journal Applied Spectroscopy, Martina Meinke and her collaborators measured the diffuse reflectance of human blood and found the spectrum which is shown below. This particular spectrum is for blood with a hematocrit (the percent of the blood's volume taken up by red blood cells) of 33% and oxygen saturation of 100%. These researchers also measured the reflectance spectrum of blood for other hematocrit values and oxygen saturation values. They found that although the spectrum slightly changes for different hematrocrit and oxygen saturation values, the overall trend shown below remains the same. Therefore, in terms of the overall trend, the image below is a good representation of the reflectance of any human's blood. (Note that even deoxygenated blood follows these trends and is dominantly red, not blue.)

blood spectrum
Diffuse reflectance of human blood with a hematocrit of 33%, oxygen saturation of 100%, and mean cell volume of 83 femtoliters. Public Domain Image, data source: M. Meinke, image source: Christopher S. Baird.

As we see in the image above, blood mostly reflects red light. Interestingly, though, blood also reflects a little bit of green light. If we shine white light (which contains all colors) onto the blood, blood looks red since it reflects so much more red light than green light. However, if we use a light source that contains all of the visible colors except red and shine it onto the blood, the blood will be green. With no red light present in the first place, the blood can't reflect any red light. The only thing left that it can reflect is the green light. The blood is therefore green. Note that this is not a trick of the eyes. The blood is literally green. In fact, human blood is always a little bit green. We usally don't notice the green color of blood because there is typically so much more red light being reflected by the blood. But if you shine a light on the blood that contains green light but no red light, the green color of blood becomes obvious.

This is exactly what happens deep in the ocean. Water is naturally very slightly blue colored because it absorbs some of the red light passing through. The deeper you go in the ocean, the less red light there is in the sunlight that reaches you. Without red color in the sunlight, only green light reflects from the blood. This fact can be startling to divers who get a cut while diving. Again, the blood does not change when in the deep ocean. Rather, the green color of blood that is always there becomes obvious once the brighter red color is no longer present. Since the green reflectance peak of blood is always there, blood can be obviously green anytime you have a light source with no red color, and not just in the deep ocean.

Topics: blood, blue, color, diving, green, red, spectrum, water