STANDARDS

CCSS: 6.RP.A.1, MP1, MP6, MP7

TEKS: 6.5A

Article Options

Presentation View
Teaching Resources

Lexile® measure

A Rainbow of Blood

Not all animals have red blood like humans . . . so what gives blood its hue?

October 2022
By Jennifer Hackett

sjharmon/Getty Images

Horseshoe crabs have blue blood. People have found that it’s useful for testing vaccines for contaminants. Watch the video to learn more!

For most animals, blood provides essential nutrients to organs, circulates oxygen throughout the body, and forms clots to close up wounds. But not all blood is the same. If you’ve ever gotten a cut, you know that humans have dark-red blood, like most mammals. But if a crustacean, insect, or worm gets a cut, you might be surprised at the color it bleeds!

Blood is made up of many different elements—it isn’t one homogenous, or uniform, fluid. Blood has four main components. There are cells that carry oxygen (called red blood cells in humans) and cells that fight infection (called white blood cells in humans). The smallest type of cell in blood, platelets, slow or stop bleeding. All these cells are carried in a fluid called plasma, which contains proteins, electrolytes, and more.

Blood is an essential part of most animals’ bodies. It brings nutrients to organs, carries oxygen throughout the body, and forms clots to close up wounds. But not all blood is the same. If you’ve ever gotten a cut, you know that humans have dark-red blood. So do most other mammals. But if a shrimp, insect, or worm gets a cut, things look different. You might be surprised at the color those animals bleed!

Blood isn’t actually one uniform fluid. Instead, it’s made up of many different elements. Blood has four main components. There are cells that carry oxygen (called red blood cells in humans). There are cells that fight infection (called white blood cells in humans). Platelets, the smallest cells in blood, slow or stop bleeding. All of these cells are carried in a fluid called plasma.

For us humans, red blood cells give blood its color—but not because the cell itself is red. “Red blood cells in humans contain a protein complex called hemoglobin,” says Jethro Forbes, an emergency and critical care veterinarian at the Cornell University College of Veterinary Medicine. “Hemoglobin is what actually binds or holds on to oxygen molecules.” This protein includes iron. When it binds with oxygen, the cells reflect red light because of the iron present, which makes blood look red. But in other animals, like the horseshoe crab, the proteins in oxygen-carrying blood cells are different. Instead of having iron in hemoglobin, horseshoe crabs have copper in hemocyanin, which makes their blood appear blue when carrying oxygen.

Human blood gets its color from red blood cells. But the cells themselves aren’t actually red. “Red blood cells in humans contain a protein called hemoglobin,” says Jethro Forbes. He’s a veterinarian at the Cornell University College of Veterinary Medicine. “Hemoglobin is what holds on to oxygen molecules.” Hemoglobin includes the element iron. Iron looks red when bound with oxygen, giving blood its hue.

But other animals, like the horseshoe crab, have different proteins in their blood cells. Instead of hemoglobin, they have hemocyanin. Instead of iron, hemocyanin contains copper. Copper looks blue when it’s connected to oxygen. That makes the horseshoe crab’s blood blue too!

Despite the shockingly different shades, all blood works pretty much the same way. Proteins like hemoglobin or hemocyanin bond with oxygen to move it through the animal’s body. If all these cells work roughly the same, why the variety in proteins and colors? Forbes and most biologists believe it’s due to evolution causing similar but different results. “As with most things in nature and animals, it has to do with surviving,” he says.

Despite the shockingly different shades, all blood works pretty much the same way. Proteins bond with oxygen to move it through the animal’s body. So why the variety? Biologists believe it’s because of how different groups of animals evolved. Each one may have had different elements in its blood to start with. But over time, they developed similar systems because those were the most effective. “As with most things in nature and animals, it has to do with surviving,” says Forbes.

COLOR CHEMISTRY

The chemicals in an animal’s blood determine what color it is. Here’s what makes each color!

Shutterstock.com

HUMANS AND OTHER ANIMALS

RED BLOOD is a result of hemoglobin, which contains the element IRON. The iron in an adult human’s hemoglobin could make a coin the size of a penny!

RED BLOOD is a result of hemoglobin, which contains the element IRON. The iron in an adult human’s hemoglobin could make a coin the size of a penny!

Shutterstock.com

SEA CUCUMBERS

YELLOW BLOOD is a result of hemovanadin, which contains the element VANADIUM. Hemovanadin is in the blood of sea cucumbers, which has 100 times more vanadium than the seawater they live in!

YELLOW BLOOD is a result of hemovanadin, which contains the element VANADIUM. Hemovanadin is in the blood of sea cucumbers, which has 100 times more vanadium than the seawater they live in!

Shutterstock.com

LEECHES

GREEN BLOOD comes from chlorocruorin, which also contains IRON. But the iron binds differently to other elements, so leeches have green blood. Some lizards also have green blood, but for a different reason: They have a buildup of biliverdin, which is a substance that makes your bruises look green!

GREEN BLOOD comes from chlorocruorin, which also contains IRON. But the iron binds differently to other elements, so leeches have green blood. Some lizards also have green blood, but for a different reason: They have a buildup of biliverdin, which is a substance that makes your bruises look green!

Timothy Fadek/Redux

HORSESHOE CRABS

BLUE BLOOD is a result of hemocyanin, which has the element COPPER. Animals with hemocyanin, like horseshoe crabs, have clear blood until it’s oxygenated, or carries oxygen. Then their blood turns blue.

BLUE BLOOD is a result of hemocyanin, which has the element COPPER. Animals with hemocyanin, like horseshoe crabs, have clear blood until it’s oxygenated, or carries oxygen. Then their blood turns blue.

Shutterstock.com

PEANUT WORMS

PURPLE BLOOD is due to hemerythrin. It also has IRON, but it binds with oxygen more strongly than hemoglobin. So it’s ideal for peanut worms living in oxygen-poor environments.

PURPLE BLOOD is due to hemerythrin. It also has IRON, but it binds with oxygen more strongly than hemoglobin. So it’s ideal for peanut worms living in oxygen-poor environments.

COLOR CHEMISTRY

The chemicals in an animal’s blood determine what color it is. Here’s what makes each color!

Shutterstock.com

HUMANS AND OTHER ANIMALS

RED BLOOD is a result of hemoglobin, which contains the element IRON. The iron in an adult human’s hemoglobin could make a coin the size of a penny!

RED BLOOD is a result of hemoglobin, which contains the element IRON. The iron in an adult human’s hemoglobin could make a coin the size of a penny!

Shutterstock.com

SEA CUCUMBERS

YELLOW BLOOD is a result of hemovanadin, which contains the element VANADIUM. Hemovanadin is in the blood of sea cucumbers, which has 100 times more vanadium than the seawater they live in!

YELLOW BLOOD is a result of hemovanadin, which contains the element VANADIUM. Hemovanadin is in the blood of sea cucumbers, which has 100 times more vanadium than the seawater they live in!

Shutterstock.com

LEECHES

GREEN BLOOD comes from chlorocruorin, which also contains IRON. But the iron binds differently to other elements, so leeches have green blood. Some lizards also have green blood, but for a different reason: They have a buildup of biliverdin, which is a substance that makes your bruises look green!

GREEN BLOOD comes from chlorocruorin, which also contains IRON. But the iron binds differently to other elements, so leeches have green blood. Some lizards also have green blood, but for a different reason: They have a buildup of biliverdin, which is a substance that makes your bruises look green!

Timothy Fadek/Redux

HORSESHOE CRABS

BLUE BLOOD is a result of hemocyanin, which has the element COPPER. Animals with hemocyanin, like horseshoe crabs, have clear blood until it’s oxygenated, or carries oxygen. Then their blood turns blue.

BLUE BLOOD is a result of hemocyanin, which has the element COPPER. Animals with hemocyanin, like horseshoe crabs, have clear blood until it’s oxygenated, or carries oxygen. Then their blood turns blue.

Shutterstock.com

PEANUT WORMS

PURPLE BLOOD is due to hemerythrin. It also has IRON, but it binds with oxygen more strongly than hemoglobin. So it’s ideal for peanut worms living in oxygen-poor environments.

PURPLE BLOOD is due to hemerythrin. It also has IRON, but it binds with oxygen more strongly than hemoglobin. So it’s ideal for peanut worms living in oxygen-poor environments.

WRITING RATIOS

Blood proteins like hemoglobin are compounds—substances made of two or more chemically combined atoms of elements—with specific ratios of different atoms. Ratios are comparisons between two or more quantities.

EXAMPLE

Hemoglobin is red when carrying oxygen. It contains a compound that has 3 hydrogen atoms for every 1 carbon atom.

PARTS: 3 hydrogen, 1 carbon

WHOLE: 3 + 1 = 4 total atoms

Part to Part

What is the ratio of hydrogen atoms to carbon atoms?
3:1

Part to Whole

What is the ratio of hydrogen atoms to total atoms in the compound?
3:4

Whole to Part

What is the ratio of total atoms in the compound to carbon atoms?
4:1

WRITING RATIOS

Blood proteins like hemoglobin are compounds—substances made of two or more chemically combined atoms of elements—with specific ratios of different atoms. Ratios are comparisons between two or more quantities.

EXAMPLE

Hemoglobin is red when carrying oxygen. It contains a compound that has 3 hydrogen atoms for every 1 carbon atom.

PARTS: 3 hydrogen, 1 carbon

WHOLE: 3 + 1 = 4 total atoms

Part to Part

What is the ratio of hydrogen atoms to carbon atoms?
3:1

Part to Whole

What is the ratio of hydrogen atoms to total atoms in the compound?
3:4

Whole to Part

What is the ratio of total atoms in the compound to carbon atoms?
4:1

Write ratios based on the atoms found in different blood proteins. Record your work and answers on our answer sheet.

Write ratios based on the atoms found in different blood proteins. Record your work and answers on our answer sheet.

A. Red hemoglobin is made up of groups of atoms called hemes. For every 1 iron atom in a heme, 4 nitrogen atoms are attached to it. What is the ratio of nitrogen atoms to iron atoms in a heme?

A. Red hemoglobin is made up of groups of atoms called hemes. For every 1 iron atom in a heme, 4 nitrogen atoms are attached to it. What is the ratio of nitrogen atoms to iron atoms in a heme?

B. What type of ratio is this?

B. What type of ratio is this?

A. For every 1 atom of vanadium in yellow-colored hemovanadin, there are 4 oxygen atoms and 2 nitrogen atoms connected to it. What is the ratio of total atoms to oxygen atoms?

A. For every 1 atom of vanadium in yellow-colored hemovanadin, there are 4 oxygen atoms and 2 nitrogen atoms connected to it. What is the ratio of total atoms to oxygen atoms?

B. What type of ratio is this?

B. What type of ratio is this?

A. When purple hemerythrin carries oxygen, every 1 oxygen atom is connected to 2 iron atoms and 1 hydrogen atom. What’s the ratio of iron atoms to oxygen atoms?

A. When purple hemerythrin carries oxygen, every 1 oxygen atom is connected to 2 iron atoms and 1 hydrogen atom. What’s the ratio of iron atoms to oxygen atoms?

B. What type of ratio is this?

B. What type of ratio is this?

A. When blue hemocyanin is carrying oxygen, each of its 2 copper atoms are connected to 2 oxygen atoms and 3 nitrogen atoms. What is the ratio of copper atoms to the total number of atoms connected to them?

A. When blue hemocyanin is carrying oxygen, each of its 2 copper atoms are connected to 2 oxygen atoms and 3 nitrogen atoms. What is the ratio of copper atoms to the total number of atoms connected to them?

B. What type of ratio is this?

B. What type of ratio is this?

Ratios & Proportions
Ratios
videos (2)
videos (2)
Skills Sheets (4)
Skills Sheets (4)
Skills Sheets (4)
Skills Sheets (4)
Games (1)
Lesson Plan (1)
Article (1)
Text-to-Speech