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Blood Color in Humans and Animals: Meaning and Function

Linda Crampton is a writer and teacher with a first-class honors degree in biology. She often writes about the scientific basis of 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.

The Reason for 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 the body. Animals may transport some substances in a different way from humans, however.

In humans, oxygenated blood is bright red and deoxygenated blood is dark red or maroon. The color is due to the presence of hemoglobin molecules in the red blood cells. Hemoglobin is a respiratory pigment. It transports oxygen to the tissue cells, which need the chemical to produce energy. Blood that is not red may indicate a health problem. Human blood may become brown or green due to the buildup of an abnormal form of hemoglobin.

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, however.

A polypeptide consists of amino acids linked together. In the color-coded illustration of a hemoglobin molecule shown above, the red polypeptide chains are known as alpha chains, the blue polypeptide chains are known as beta ones, and the green units are the heme groups.

Red Blood

The most common blood color in humans and animals is red. Hemoglobin is present in humans, most other vertebrates, and some invertebrates as well.

Pigment Structure

A hemoglobin molecule is a complex structure made of four globular polypeptide chains that are joined together, as shown in the illustration above. Two of the chains are alpha ones and the other chains are beta ones. The alpha and beta chains have a different sequence of amino acids. A heme group is embedded in each chain, or subunit, of the molecule. The heme groups are the pigmented portions of the hemoglobin molecule and contain iron. The iron joins reversibly with oxygen.

Location of the Pigment

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 the molecules gives the blood a red appearance.

Functions 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 tissue 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. The 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 other 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 other changes. Veins are usually colored blue in illustrations.

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 those in our hands, they do appear to be blue in color. The blue appearance is caused the behavior of light as it enters and leaves the body through the skin and not by the the blood itself.

"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 in different ways 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

Features of 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. The high concentration of methemoglobin causes the blood to appear red brown or even chocolate brown.

Methemoglobinemia is sometimes an inherited condition. It may also be caused by chemicals in medications or food. This form of the disorder 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 the disease can be treated successfully, often by methylene blue administration by a medical professional.

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.


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 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, reportedly caused one case of green blood discovered by doctors. Sumatriptan is sometimes known as Imitrex. It belongs to a group of chemicals known as sulfonamides.

Unlike methemoglobinemia, sulfhemoglobinemia can't be treated with a medication that returns the hemoglobin to normal. The abnormal pigment is gradually eliminated as old red blood cells are broken down and new ones with new hemoglobin are made, provided the cause of the damaged pigment 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.

Green Blood in a Vertebrate and Invertebrates

Vertebrates generally have red blood, but there are some exceptions. One genus of skink (Prasinohaema) has green blood and is given the name green-blooded skink. Like other vertebrates, green-blooded skinks do have hemoglobin in their blood. The blood also contains a very high concentration of biliverdin, however.

Biliverdin is a green pigment produced from the breakdown of hemoglobin. Its main location in most vertebrates is in bile, a 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 or in humans.

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.

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 it moves through a body cavity called a hemocoel. The fluid in the circulatory system is technically known as hemolymph. Outside of science, however, it's often called blood.

The Open Circulatory System in Insects

Blue Hemolymph

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.

The insect in the photo above is a Peleides blue morpho butterfly (Morpho peleides). It's blue on its dorsal (upper) surface and brown on its ventral one.

Yellow Hemolymph

Insects are arthropods with pale yellow, pale green, or colorless hemolymph. A squashed mosquito may release red blood, but this 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 liquid 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 vanabins actually transport oxygen in a sea cucumber's body. At least some species of sea cucumber have hemoglobin in their circulatory fluid.

Orange and Violet Hemolymph

Like other insects, cockroaches have tracheae that transport oxygen and have no respiratory pigment in their hemolymph. The liquid is usually colorless. Females that are producing eggs may have pale orange hemolymph, however. Inside their bodies, an organ called the fat body makes an orange protein called vitellogenin. This 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

Colorless Blood in Icefish

Icefish generally live in the Antarctic and belong to the family Channichthyidae. They are also called crocodile fish due to the shape of their long snout and white-blooded fish because their colorless blood has no red blood cells and no respiratory pigment. Oxygen is transported in the blood plasma of the animals. Icefish are the only vertebrates with colorless blood.

The fish have a number of adaptations that allow them to live successfully in cold water. Oxygen dissolves better in cold water than warm water, though this property on its own is not sufficient to keep the fish alive. The animals have a large heart that pumps a lot of blood with each beat. They also have a larger blood volume than fish of a comparable size that have red blood as well as more blood vessels in their skin. These vessels absorb some oxygen, though the icefish does have gills for absorbing oxygen as well.

An ocellated icefish, or Chionodraco rastrospinosus

An ocellated icefish, or Chionodraco rastrospinosus

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 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.


This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

Questions & Answers

Question: I'm doing a poster on why humans have red blood and why spiders have blue blood. Could you give more information about spider blood?

Answer: Hemocyanin is an example of a metalloprotein (a protein that contains a metal). In some countries, its name is spelled haemocyanin. Oxygenated hemocyanin in spider hemolymph absorbs all colours of light, except blue, which it reflects to our eyes. This makes the hemolymph look blue. Without oxygen, the hemolymph is colourless.

Two copper atoms in the hemocyanin join to one oxygen molecule. The copper is actually in the form of the copper (I) ion (one which has a +1 charge) when it isn’t bound to oxygen and the copper (II) ion (one which has a +2 charge) when it is bound to the oxygen.

Question: The nurse taking my blood said that high triglycerides cause the blood to have a milky appearance and liver problems cause a yellow cast. Is this true?

Answer: Your nurse is correct with respect to the potential effect of high triglycerides in the blood, plasma, or serum. (Plasma is blood with the cells removed. Serum is plasma with clotting factors removed.) Triglycerides are a type of fat. A very high triglyceride level can cause blood, plasma, or serum to have a milky appearance. Some precautions are needed in an interpretation of the colour change, however. A doctor must be consulted. More than one factor can cause a particular change in blood. A doctor would likely perform other tests to diagnose the cause of a colour change and not rely entirely on the liquid’s appearance.

Jaundice is a disorder that is also known as icterus. It’s sometimes (but not always) caused by liver problems. The concentration of a yellow substance in the blood called bilirubin increases in jaundice. Bilirubin collects in the skin and the whites of the eyes, causing these areas to turn yellow. Perhaps this is what your nurse meant when he or she mentioned a yellow cast. In addition, bilirubin collects in the urine during jaundice, causing the fluid to become dark. I have never read anything about the blood developing a yellow cast, however, despite its increased level of bilirubin. You should ask your doctor whether this happens.

Question: What is the color of cow and bull blood?

Answer: Cattle are mammals, like us, so they have red blood containing hemoglobin. The blood of bulls generally has a higher concentration of red blood cells and hemoglobin than the blood of cows.

© 2012 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on July 07, 2018:

That's a very interesting idea for using the knowledge of different blood colours! Thanks for the comment, Fern.

Fern deLange from British Columbia on July 07, 2018:

Thank you so much for creating this article! It will be very useful for creating characters with alternative blood colours, and knowing the colours of their blood in their deoxygenated states will be incredibly useful for painting the colour zones of the characters.

Linda Crampton (author) from British Columbia, Canada on January 15, 2018:

Thank you very much for the kind comment, Alyssa. I appreciate your visit a great deal.

Alyssa from Ohio on January 15, 2018:

This is an amazing article! It's interesting to learn why different species of animals have different blood colors, and how we can use that to not only better understand life, but also work to improve quality of life. Thank you so much for sharing this!

Linda Crampton (author) from British Columbia, Canada on November 23, 2017:

Hi, Missy. Since your doctor took the blood sample, you should ask him or her about your son's blood colour. The doctor will be able to give you information about the condition of your son's blood.

Linda Crampton (author) from British Columbia, Canada on November 23, 2017:

Hi, Kristen. Thank you for the comment. One example related to your question is the use of methylene blue to remove excess methemoglobin from the blood. This can change the colour of the blood from an abnormal dark red-brown colour to the normal red colour.

Missy on November 23, 2017:

Took my 14 year old son to Dr and his blood appeared to be like a deep chalky red what can this mean

Kristen on November 21, 2017:

Thank you for your amazing article! I have a question, however. Can substances--such as medical treatments or injections--change the color of your blood? And if so, what color?

Linda Crampton (author) from British Columbia, Canada on November 16, 2017:

I've never heard of white blood. Pus exuding from an open wound can be white, though. It's composed of dead white blood cells, bacteria, other cell types, proteins, and fluid.

Fazal on November 12, 2017:

Great. Wat about white color in a human blood?

Linda Crampton (author) from British Columbia, Canada on July 02, 2017:

Hi. The term "golden blood" doesn't refer to a colour. It's used to describe a rare blood type in which the red blood cells have none of the rhesus antigens on their surface. This antigen family contains many members, so its absence in golden blood is interesting to researchers.

vipul on June 24, 2017:

Is golden colour blood in humans? Please tell me about golden blood.

Linda Crampton (author) from British Columbia, Canada on April 16, 2017:

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

Zachary Turry on April 16, 2017:

This article helped me learn about the different color of blood THANK YOU

Linda Crampton (author) from British Columbia, Canada on January 21, 2017:

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.

Linda Crampton (author) from British Columbia, Canada on January 18, 2017:

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.

Linda Crampton (author) from British Columbia, Canada on October 19, 2016:

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

Linda Crampton (author) from British Columbia, Canada on September 28, 2016:

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

dhivya on September 28, 2016:

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

Linda Crampton (author) from British Columbia, Canada on July 18, 2015:

Thank you for the visit and the thanks, MUKI.

MUKI on July 18, 2015:

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

Muki on July 18, 2015:

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

Linda Crampton (author) from British Columbia, Canada on July 16, 2015:

I'm glad the information helped you, CUTIE.

CUTIE on July 16, 2015:

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

Linda Crampton (author) from British Columbia, Canada on April 09, 2014:

Thank you very much, taib wechuli.

taib wechuli on April 09, 2014:

i really apreciate your work.good job

Linda Crampton (author) from British Columbia, Canada on January 22, 2014:

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

luisj305 from Florida on January 22, 2014:

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.

Linda Crampton (author) from British Columbia, Canada on July 06, 2013:

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!

Robert on July 06, 2013:

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

Linda Crampton (author) from British Columbia, Canada on May 05, 2013:

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

Susan Bailey from South Yorkshire, UK on May 05, 2013:

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

Linda Crampton (author) from British Columbia, Canada on March 18, 2013:

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

Leah Kennedy-Jangraw from Massachusetts on March 18, 2013:

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!

Linda Crampton (author) from British Columbia, Canada on December 05, 2012:

Thanks for the visit and the comment, Eddy.

Eiddwen from Wales on December 05, 2012:

Very interesting indeed.


Linda Crampton (author) from British Columbia, Canada on December 04, 2012:

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

Dianna Mendez on December 04, 2012:

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.

Linda Crampton (author) from British Columbia, Canada on December 03, 2012:

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

ignugent17 on December 03, 2012:

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

Thanks! :-)

Linda Crampton (author) from British Columbia, Canada on December 02, 2012:

Thanks, Deb. I appreciate your visit and comment!

Linda Crampton (author) from British Columbia, Canada on December 02, 2012:

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

Deb Hirt from Stillwater, OK on December 02, 2012:

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!

drbj and sherry from south Florida on December 02, 2012:

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.

Linda Crampton (author) from British Columbia, Canada on December 02, 2012:

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

Linda Crampton (author) from British Columbia, Canada on December 02, 2012:

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

Bill Holland from Olympia, WA on December 02, 2012:

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.

Martie Coetser from South Africa on December 02, 2012:

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.

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