Blood Color in Humans and Animals: Meaning and Function
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 materials 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 indicates 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.
The most common blood color in humans and animals is red. Hemoglobin is present in humans, most other vertebrates, and some invertebrates as well.
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 molecule and contain iron.
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.
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 results from the behavior of light as it enters and leaves the body, however, and not from the color of the blood.
"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.
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.
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
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.
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.
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.
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.
Methemoglobinemia from the U.S. National Library of Medicine
A case of sulfhemoglobinemia as described by the BBC
Lizards with green blood from the Smithsonian magazine
Differences between insect blood and ours from Scientific American
Components of the blood (including invertebrate respiratory pigments) from the Concepts in Biology textbook by Charles Monar and Jane Gair
Translucent blood in Antarctic icefish from EarthSky
Swaminathan, A., Lucas, R. M., Dear, K., & McMichael, A. J. (2014). Keyhole limpet haemocyanin - a model antigen for human immunotoxicological studies. British journal of clinical pharmacology, 78(5), 1135-42.
Questions & Answers
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?
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.Helpful 11
© 2012 Linda Crampton