We see that the blood flowing from the body is red, but sometimes the blood vessels seen through the skin are blue. Some people say that venous blood is blue due to lack of oxygen. Some people say that it is the color of blood vessels. Is that right?
This is about hemoglobin in the blood. Hemoglobin contains a red compound called heme, which plays a crucial role in the transport of oxygen in the blood. When the iron atoms in the heme are combined with oxygen, the resulting molecules transport oxygen from the lungs to the rest of the body.
The specific color that chemical substances present in front of our eyes depends on the wavelength of the light they reflect. Hemoglobin, which is combined with oxygen, absorbs blue-green light, which means that it reflects red-orange light into our eyes so that it appears red. Therefore, when oxygen is combined with iron in the blood, the blood becomes fresh cherry red. If there is no oxygen, the blood will appear dark red.
Carbon monoxide can also be combined with hemoglobin, and the affinity of carbon monoxide and hemoglobin is about 200 times greater than that of oxygen and hemoglobin. Therefore, when the concentration of carbon monoxide reaches a certain concentration, oxygen cannot be combined with hemoglobin. After the hemoglobin loses its ability to carry oxygen, the human body will experience symptoms of poisoning and even death. Carbon monoxide hemoglobin has a more vivid red color than oxyhemoglobin, so abnormal cherry red blood is one of the symptoms of carbon monoxide poisoning. Severe patients look flushed and their lips are cherry red.
Sometimes it is seen through the skin that the blood in the blood vessels is blue. Some people think that because the blood in veins lacks oxygen, they are blue instead of red; or the blood vessels themselves are green. But in fact, this argument is wrong. Blue blood vessels are only an optical illusion. If the blood vessels have a certain depth under the skin, your eyes see more blue light than red light because skin tissue and blood absorb some of the red wavelengths, so what the naked eye sees is blue-blue.
However, blue blood exists in the animal world, such as squid and horseshoe crabs. The blood of other animals is also green, transparent or even purple because the animals use different molecules to transport oxygen, not hemoglobin.
With these exceptions, the blood of most animals is also red, but it is not necessarily the same as the red color of human blood, because different species of hemoglobin also have differences.
As blood flows out of the body, it becomes darker, and hemoglobin breaks down into a complex called methemoglobin. Over time, the color of blood will become darker and produce another complex called the high iron iron chromogen. Based on this continuous change in color and chemical substances, forensics can determine when the blood appears at the crime scene.