Blood Colors

First of all, we want to wish all of our readers a Happy (and safe) Halloween! Now, onto the science of blood.

Some of the most iconic monsters of all time seem obsessed with blood. Of course, vampires are the most famous among mythical creatures, but sharks can smell blood in the water from miles away, and vultures and ravens are attracted to the smell of blood and decaying flesh on land. All animals, with the exception of some very simple invertebrate animals like jellyfish, coral, and flatworms, have blood. An average human will have between 1 and 1.5 gallons of blood in their body at any time, and blood accounts for roughly 7% of the person's weight. It's an essential, valuable thing.

In general, human blood is made up of four major parts: white blood cells, plasma, platelets, and red blood cells. White blood cells, or leukocytes (luke-o-cites), are part of your body's immune system, and they are responsible for responding to microscopic threats that your body faces from bacteria, parasites, and infections. Platelets, or thrombocytes, are the small cells that stick together to form scabs and stop vessels from bleeding. They are the reason why all your blood doesn't spill out when you get a cut! Plasma is the colorless liquid that the other components are suspended in, and is mostly made of water.

Red blood cells, photographed with a scanning electron
microscope. Photo by the Wellcome Trust/Marc Turner.

Red blood cells are also called erythrocytes (air-ith-throw-cites), and they give blood its red color. Their job is to carry oxygen to every cell in your body. The cells then use the oxygen to create food and energy. The red blood cells also take waste away from the cells. As the red blood cells pass through organs like the liver and the kidneys, the waste is filtered out and eventually expelled from the body. Red blood cells are born in a person's bone marrow, and every cell will live roughly 120 days (about 4 months). Once they die, the spleen filters them out. The spleen also removes any red blood cells that are misshapen or damaged.

Of course, the color most associated with blood is red (I mean, it's right there in the name of the cells!). But blood actually comes in a rainbow of different colors, and it all has to do with the types of chemicals that it contains. Most vertebrates (animals with backbones) have red blood because our red blood cells contain hemoglobin, an iron-based protein. Each hemoglobin protein is made up of individual hemes, which bind iron particles. The iron particles in turn bind oxygen, which is then carried to the rest of the body. A deficiency in iron is called anemia, and is actually a fairly common deficiency in America. Since a low iron level makes it harder for blood to carry oxygen to cells, a person with anemia may become tired and short of breath more easily than someone with higher iron levels.

The different colors of blood. Image by Compound Chem.

Red blood is just one color though. Animals like horseshoe crabs have blue blood because instead of hemoglobin, they carry a protein called hemocyanin. This protein contains copper in place of iron, but it functions in much the same way. When hemocyanin-rich blood is carrying no oxygen, it is actually colorless! These animals have evolved a different type of blood circulation system that doesn't require as high of an efficiency rate, so copper sufficed.

Green blood is also seen in nature, but almost exclusively in worms and leeches. The protein that makes blood green is called chlorocruorin. Chlorocruorin also contains iron in its center, but one branch of the protein is replaced with a different chemical structure, which alters the entire protein. There is one lizard, aptly called the green-blooded skink, whose blood is indeed green. This lizard actually has true hemoglobin in its blood, but when its cells die, its body cannot break the protein down as far as our bodies can. The result is that the lizard's blood has a higher concentration of a by-product called biliverdin, which gives the lizard's blood a green color.

The green-blooded skink. Photo by Christopher Austin,
Louisiana State University Museum of Natural Science

One last shade of blood is bright violet-pink, which is the rarest color of all, occurring only in some species of marine worms. This blood's color is from a protein called hemerythrin, which is made of individual chemical units which all contain iron. This protein can only carry about 25% of the oxygen that hemoglobin could carry, unfortunately for the worms!

As much as modern science has learned about blood over centuries of study, the one thing no one can yet do is manufacture blood. Since blood is such a vital resource, and because it cannot be created, the only source of blood for people is other people. This is why blood drives are so necessary for people suffering from diseases or recovering from major accidents. If you are able and willing, please try to donate blood soon. Perhaps you can even make a joke about how the blood drive workers just wanted to suck out your blood!

Sources:
http://science.sciencemag.org/content/192/4237/335
https://www.compoundchem.com/2014/10/28/coloursofblood/
http://scienceline.ucsb.edu/getkey.php?key=2419
http://idahoptv.org/sciencetrek/topics/blood/facts.cfm
http://www.chp.edu/our-services/transplant/liver/education/organs/spleen-information