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 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.
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.
Function of Hemoglobin
In the lungs oxygen that we inhale binds to the iron in the hemoglobin molecules, causing 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.
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 the colors of the visible spectrum, which have different wavelengths and energies. These 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.
Brown Blood
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 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.
Green Blood
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 only exist for only about 120 days.) If a person has severe sulfhemoglobinemia he or she may need a blood transfusion.
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 and is normally found in bile, a green secretion produced by the liver to emulsify fats in the intestine and make them easier to digest. In the green-blooded skink the biliverdin in the blood reaches levels that would be toxic in other lizards.
Blue Blood
The blood 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, and 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.
The circulating liquid in many invertebrates is technically known as hemolymph instead of blood. Hemocyanin is found in the hemolymph instead of being trapped in cells. It’s the respiratory pigment in molluscs (such as snails, slugs, clams, octopuses and squids), and in some arthropods (such as crabs, lobsters and spiders).
Yellow Blood
Insects have 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 which is 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.
Orange 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.
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.
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