Human blood always has a rich red color. This is due to the fact that it contains the protein hemoglobin, which includes a red-colored complex compound called “heme”.
Heme contains an iron atom that combines with oxygen. This compound plays a key role in transporting oxygen from the lungs through the circulatory system to other organs.
Chemicals appear one color or another in our eyes depending on the wavelength of light they reflect.
Hemoglobin, combined with oxygen, absorbs blue-green rays and reflects red-orange rays, which we perceive as red. Therefore, oxygen-saturated arterial and capillary blood is bright red, and venous blood, which has little oxygen, is dark burgundy.
Blood that has leaked out of the body becomes darker and darker over time as it dries, because the hemoglobin in it breaks down and turns into methemoglobin. The color of dried blood continues to change further, becoming darker and darker due to another compound, hemichrome.These continuous color and chemical changes allow forensic scientists to determine when blood stains were left at a crime scene.
Why are veins blue
In people with pale skin, the veins appear blue. This is not because the blood flowing through them is blue, nor is it because the blood returning through the veins to the lungs contains little oxygen. Human blood never, under any circumstances, turns blue or blue.
The blue color of the veins is nothing more than an optical illusion. The fact is that the light of the red-orange spectrum, which has a long wavelength, is able to penetrate into tissues to a greater depth than the light of the blue-green part of the spectrum. When red light reaches the veins located under the skin, it is absorbed by hemoglobin and is not reflected into our eyes.
Therefore, we see only blue light, which is easily reflected from the surface of the skin.
It should be said that in the animal kingdom there are examples of blue, green, colorless and even purple blood. The color of the blood depends on the substance that carries oxygen around the body. For example, squid and horseshoe crabs have blue blood because it is copper, not iron, that is responsible for oxygen transfer, as in humans.