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Blood Color in Humans and Animals - Meaning and Function

Updated on April 04, 2016
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Linda Crampton is a teacher with an honours degree in biology. She enjoys writing about human biology and the science of health and disease.

Not all blood is red. A robber crab has a molecule called hemocyanin in its blood. Hemocyanin is blue in its oxygenated form.
Not all blood is red. A robber crab has a molecule called hemocyanin in its blood. Hemocyanin is blue in its oxygenated form. | Source

The Meaning of Blood Color

Human blood is a beautiful red color, but the blood of some animals - and of humans under certain conditions - is a different color. The function of all blood is to transport vital substances around a body. Animal blood may perform some of its transport jobs in a different way from human blood, however.

In humans, oxygenated blood is bright red and deoxygenated blood is dark red or maroon. The color is produced by the hemoglobin molecules in our red blood cells. A color other than red indicates a health problem. For example, human blood may become brown or green due to the buildup of an abnormal form of hemoglobin. Hemoglobin is a type of respiratory pigment - a colored molecule that transports oxygen around the body to the cells, which require oxygen to produce energy.

Animals may have red, blue, green, yellow, orange, violet or colorless blood. Some have hemoglobin like us, some have different respiratory pigments and some have no respiratory pigments at all. All animals have developed a method to transport oxygen, even if they don't have respiratory pigments.

A hemoglobin molecule: the red and blue sections are the protein subunits while the green sections are the heme groups
A hemoglobin molecule: the red and blue sections are the protein subunits while the green sections are the heme groups | Source

Red Blood

The most common blood color in humans and animals is red. This color is mainly due to the presence of hemoglobin, a red molecule which is present in humans, most other vertebrates and some invertebrates too.

Hemoglobin Structure

A hemoglobin molecule is a complex structure made of four globular protein subunits that are joined together. A heme group is embedded in each subunit. The heme groups are the pigmented portions of the hemoglobin molecule and contain iron.

Location of Hemoglobin

Hemoglobin is located in the red blood cells of humans. There are between 4 and 5 million red blood cells in each cubic millimeter (or microliter) of an adult female's blood and between 5 and 6 million in the same volume of an adult male's blood. Each red blood cell, or erythrocyte, contains about 270 million hemoglobin molecules. The high concentration of hemoglobin molecules gives the blood a red appearance.

Human red blood cells, or erythrocytes
Human red blood cells, or erythrocytes | Source

Function of Hemoglobin

In the lungs, oxygen that we inhale binds to the iron in the hemoglobin molecules. This causes the hemoglobin to become bright red in color. The oxygenated hemoglobin (or oxyhemoglobin) is transported from the lungs through the arteries, into the narrower arterioles and then into the tiny capillaries. The capillaries release the oxygen to the cells, which use it to produce energy.

When hemoglobin gives up its oxygen to the cells it changes from bright red to a dark red or maroon color. This dark red, deoxygenated hemoglobin is transported back to the lungs through the venules and the veins to pick up a fresh supply of oxygen.

Veins on the back of the hand show up more clearly as we age due to tissue loss and changes. Veins are usually colored blue in illustrations.
Veins on the back of the hand show up more clearly as we age due to tissue loss and changes. Veins are usually colored blue in illustrations. | Source

Color of Blood in Veins

All blood in the body is red, although the shade of red varies. Blood in veins isn't blue, even though in illustrations of the circulatory system the veins are traditionally colored blue. When we look at the veins close to the surface of our body, such as veins in our hands, they do appear to be blue in color. The blue appearance results from the behavior of light as it enters and leaves the body, however, and not from the color of the blood in the veins.

"White" light from the sun or an artificial light source is a mixture of all of the colors in the visible spectrum. The colors have different wavelengths and energies. The different wavelengths are affected differently as they hit the skin and the cells under the surface layer of the skin. Light that hits the veins and their deoxygenated blood and then emerges to reach our eyes is more likely to be in the high-energy blue region of the spectrum than in the low-energy red region of the spectrum. Therefore the veins look blue to us.

Methemoglobinemia After Benzocaine Treatment for Sore Gums

Brown Blood and Methemoglobinemia

Methemoglobinemia is a disorder in which too much methemoglobin is made. Methemoglobin has a chocolate-brown color. It's present in everyone's blood but is normally at a very low level. In a methemoglobin molecule the iron has been changed from a form that has a +2 charge to a form that has a +3 charge. When the iron is in this form, hemoglobin can't transport oxygen and the cells can't make enough energy.

Methemoglobinemia is sometimes an inherited condition. It may also be caused by chemicals in medications or food. This type of methemoglobinemia is said to be acquired and is more common than the inherited condition. Examples of chemicals that can increase the amount of methemoglobin include benzocaine (an anesthetic), benzene (which is also a carcinogen), nitrites (which are added to deli meats to prevent them from spoiling) and chloroquine (an antimalarial drug). Natural nitrates in foods can cause methemoglobinemia in babies if they are eaten in excess.

Symptoms of acquired methemoglobinemia may include fatigue, lack of energy, headache, shortness of breath and a bluish color to the skin (cyanosis). Most forms of methemoglobinemia can be treated successfully, often by methylene blue administration, although unfortunately there is one type of inherited methemoglobinemia that is difficult to treat.

Broccoli is a nutritious food, but it's high in natural nitrates which may contribute to methemoglobinemia in some people.
Broccoli is a nutritious food, but it's high in natural nitrates which may contribute to methemoglobinemia in some people. | Source

Green Blood - Sulfhemoglobinemia

In humans, a rare condition called sulfhemoglobinemia causes the blood to appear green. In this condition sulfur has joined to the hemoglobin molecules, forming a green chemical called sulfhemoglobin. The altered hemoglobin molecule can't transport oxygen.

Sulfhemoglobinemia is usually caused by exposure to high doses of certain medications and chemicals. For example, a long-term overdose of sumatriptan, a migraine medication, can lead to the condition. Sumatriptan is sometimes known as Imitrex.

Unlike methemoglobinemia, sulfhemoglobinemia can't be treated with a medication that returns the hemoglobin to normal. The abnormal hemoglobin will gradually be eliminated as old red blood cells are broken down and new ones with new hemoglobin are made, provided the cause of the damaged hemoglobin is removed. (Red blood cells exist for only about 120 days.) If a person has severe sulfhemoglobinemia he or she may need a blood transfusion.

Like broccoli, the beet or beetroot is high in natural nitrates.
Like broccoli, the beet or beetroot is high in natural nitrates. | Source

Green-Blooded Skink and Invertebrates

Vertebrates have red blood, but there is one exception. There is one genus of skink (Prasinohaema) that has green blood and is given the name green-blooded skink. Like other vertebrates, green-blooded skinks do have hemoglobin in their blood. However, the blood also contains a very high concentration of biliverdin. Biliverdin is a green pigment produced from the breakdown of hemoglobin. It's normally found in bile, a green secretion produced by the liver. Bile emulsifies fats in the small intestine and makes them easier to digest. In the green-blooded skink, the biliverdin in the blood reaches levels that would be toxic in other lizards.

Some members of the phylum Annelida (segmented worms and leeches) contain a green respiratory pigment called chlorocruorin. Blood containing chlorocruorin may be green but isn't necessarily so. Some annelids with the pigment also contain hemoglobin, which masks the green color.

Snail blood contains hemocyanin.
Snail blood contains hemocyanin. | Source

Members of the phylum Arthropoda and the phylum Mollusca have an open circulatory system. In this system, blood travels through vessels during only part of its journey around the body. The rest of the time the blood moves through a body cavity called a hemocoel. The fluid in the circulatory system is technically known as hemolymph instead of blood.

The Open Circulatory System in Insects

Blue Blood

The blood (hemolymph) of some invertebrates contains hemocyanin instead of hemoglobin. Like hemoglobin, hemocyanin transports oxygen and is a protein that contains a metal. However, hemocyanin contains copper instead of iron. It's blue in its oxygenated form and colorless in its deoxygenated form. A hemocyanin molecule contains two copper atoms, which together bind to one oxygen molecule.

Hemocyanin is the respiratory pigment in molluscs (such as snails, slugs, clams, octopuses and squids), and in some arthropods (such as crabs, lobsters and spiders). The pigment is found in the liquid hemolymph instead of being trapped in cells.

Insects have colorless, pale yellow or pale green blood.
Insects have colorless, pale yellow or pale green blood. | Source

Yellow Blood

Insects are arthropods with pale yellow, pale green or colorless blood. A squashed mosquito may release red blood, but this blood comes from the animal or human that provided the mosquito's last meal.

Oxygen is transported around an insect's body in a network of tubes known as the tracheal system. The hemolymph doesn't transport oxygen and therefore doesn't need respiratory pigments. The pale colors which are sometimes seen in the hemolymph are thought to be due to the presence of pigmented food molecules that have entered the hemolymph.

Sea cucumbers extract vanadium from sea water and concentrate it in their bodies. The vanadium is used to make proteins called vanabins, which become yellow when they're oxygenated. However, scientists don't know whether vanadins actually transport oxygen in a sea cucumber's body. At least some species of sea cucumber have hemoglobin in their circulatory fluid.

A sea cucumber
A sea cucumber | Source

Orange and Violet Blood

Like other insects, cockroaches have tracheae that transport oxygen and have no respiratory pigment in their hemolymph. The hemolymph is usually colorless. Females that are producing eggs may have pale orange hemolymph, however. In their bodies an organ called the fat body makes an orange protein called vitellogenin, which gives rise to a major egg yolk protein called vitellin. Vitellogenin is secreted into the hemolymph, giving it a slight color.

Some marine invertebrates have hemerythrin as a respiratory pigment. This pigment is colorless when deoxygenated and pink-violet in color when oxygenated.

A Cuttlefish With Hemocyanin and Other Interesting Pigments

Respiratory Pigment Research

It's interesting that different species have developed different solutions to the problem of distributing oxygen throughout the body. Scientific research in this area is useful because it helps us to understand life on Earth better. In addition, researchers are discovering that some animal respiratory pigments have benefits for humans. For example, keyhole limpet hemocyanin (KLH) has been found to stimulate the activity of our immune systems and is added to some vaccines for this reason. It will be interesting to see what future research reveals about respiratory pigments.

© 2012 Linda Crampton

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    • MartieCoetser profile image

      Martie Coetser 4 years ago from South Africa

      Very interesting, useful and well-presented hub about the color of blood. Thank you, Alicia. I love the way you present the most interesting information.

    • billybuc profile image

      Bill Holland 4 years ago from Olympia, WA

      Great information! Having taught science, I knew most of this, but I'm willing to bet many who read this will find this to be new information that they were not aware of.

    • AliciaC profile image
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      Linda Crampton 4 years ago from British Columbia, Canada

      Thank you very much, Martie. I appreciate your comment.

    • AliciaC profile image
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      Linda Crampton 4 years ago from British Columbia, Canada

      Hi, Bill. Thank you very much for the visit and comment.

    • drbj profile image

      drbj and sherry 4 years ago from south Florida

      So blood, Alicia, can be so many colors? Who knew? Fascinating discourse, m'dear, which I shall save and refer to if anyone should need a 'bloody' answer. Thank you for your in-depth and interesting research.

    • aviannovice profile image

      Deb Hirt 4 years ago from Stillwater, OK

      There is a lot of great information here. I never realized that there could be more than one color to blood. This is very well done!

    • AliciaC profile image
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      Linda Crampton 4 years ago from British Columbia, Canada

      Thank you for the comment, drbj. The color of blood is an interesting topic!

    • AliciaC profile image
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      Linda Crampton 4 years ago from British Columbia, Canada

      Thanks, Deb. I appreciate your visit and comment!

    • ignugent17 4 years ago

      Great information. This is really interesting. It is always nice to know something new. Now I know the different color of the blood.

      Thanks! :-)

    • AliciaC profile image
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      Linda Crampton 4 years ago from British Columbia, Canada

      Hi, ignugent17. Thank you very much for the comment.

    • teaches12345 profile image

      Dianna Mendez 4 years ago

      What a fascinating topic, Alicia. I am continually impressed by your articles. Always find them so interesting and well written. I have learned much today.

    • AliciaC profile image
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      Linda Crampton 4 years ago from British Columbia, Canada

      Thank you very much for the lovely comment, Dianna! I appreciate it.

    • Eiddwen profile image

      Eiddwen 4 years ago from Wales

      Very interesting indeed.

      Eddy.

    • AliciaC profile image
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      Linda Crampton 4 years ago from British Columbia, Canada

      Thanks for the visit and the comment, Eddy.

    • thebiologyofleah profile image

      thebiologyofleah 3 years ago from Massachusetts

      Really interesting article, I enjoyed the mix of the basic physiology along with the various human pathologies as well as a look at blood color in lower species. Thanks for sharing!

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you very much for the comment, thebiologyofleah. It's nice to meet you!

    • Sue Bailey profile image

      Susan Bailey 3 years ago from South Yorkshire, UK

      Amazing hub. Very interesting content. Voted up and shared.

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you, Sue!! I appreciate your comment, the vote and the share very much.

    • Robert 3 years ago

      Thank you so much for this interesting article. Are these all known colours of blood or are there even more colours?

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Hi, Robert. Thanks for the comment. These are all the colors that I know of, but there may be others too. The living world is varied and amazing!

    • luisj305 profile image

      luisj305 3 years ago from Florida

      Learned lots with this hub, really interesting. I would be amazed if I were to ever bleed out green or some other color besides red.

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thanks for the comment, luisj305. I would be amazed if I bled green blood, too!

    • taib wechuli 2 years ago

      i really apreciate your work.good job

    • AliciaC profile image
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      Linda Crampton 2 years ago from British Columbia, Canada

      Thank you very much, taib wechuli.

    • CUTIE 18 months ago

      Thanks for the above information. It helped us a lot in our science project. THANKS.......................

    • AliciaC profile image
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      Linda Crampton 18 months ago from British Columbia, Canada

      I'm glad the information helped you, CUTIE.

    • Muki 18 months ago

      Thank u very much for the information,without this what would have happened?

    • MUKI 18 months ago

      Thank you very much for the information.Apart from doing our project it helped me to know a lot more.THANX once again.....:)

    • AliciaC profile image
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      Linda Crampton 18 months ago from British Columbia, Canada

      Thank you for the visit and the thanks, MUKI.

    • dhivya 3 months ago

      thanks you alot for the detailed information its very helpful to understand and helps me more for my presentation thank you again

    • AliciaC profile image
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      Linda Crampton 3 months ago from British Columbia, Canada

      Thank you, dhivya. I'm glad the article was helpful.

    • Charlie 5th grader 3 months ago

      This helped me so much in my science progect.

    • AliciaC profile image
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      Linda Crampton 3 months ago from British Columbia, Canada

      I'm glad that the article helped you, Charlie. Thanks for the comment.

    • christy 2 days ago

      hi linda i just wanna know something

      i observed complete complete green colour in my blood last day in my slide when i was observing for wbc in microscope hope u could help me with it

    • AliciaC profile image
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      Linda Crampton 2 days ago from British Columbia, Canada

      This is something you need to ask your doctor about, christy. The slide may have been contaminated with a chemical or there may be a medication or internal condition in your body that is causing the green color. I can't diagnose an individual's problem, since I'm not a doctor. Please see your physician as soon as possible. I expect he or she will be able to help you.

    • Prabhat 33 hours ago

      Insects name having yellow blood

    • AliciaC profile image
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      Linda Crampton 2 hours ago from British Columbia, Canada

      Hi. Prabhat. Insects generally have colorless blood since they have no respiratory pigment. The blood may sometimes appear pale yellow, which is believed to be due to the presence of colored molecules from food. The presence of the yellow color depends on what the insect has been eating.

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