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Why is blood red and not any other color?

Blood appears red because of the heme group found in hemoglobin. Hemoglobin is an iron-containing protein in red blood cells that carries oxygen from the lungs to tissues and organs and transports carbon dioxide back to the lungs. The heme group, which contains iron, gives blood its red color when oxygenated.

What is hemoglobin?

Hemoglobin is a protein found in red blood cells that helps carry oxygen from the lungs to the rest of the body. It makes up about 34-36% of the total content of red blood cells and gives blood its red color. Hemoglobin has a quaternary structure made up of four subunits – two alpha subunits and two beta subunits. Each subunit contains a heme group.

The role of heme in hemoglobin

The heme group is responsible for the red color of blood. Heme contains iron in the Fe2+ (ferrous) state when hemoglobin is oxygenated. It is this form of iron that gives blood its distinct red color. The heme group forms four reversible bonds with oxygen molecules, allowing each hemoglobin molecule to carry four oxygen molecules.

How hemoglobin binds and releases oxygen

When oxygen binds to the iron atom in heme, it causes a conformational change in the hemoglobin protein structure. This allows hemoglobin to release oxygen in areas of the body where oxygen levels are low, such as in active muscles. The binding and releasing of oxygen to hemoglobin accounts for blood’s ability to change color from bright red when oxygenated to a more purplish red when deoxygenated.

The oxygen dissociation curve

The binding and releasing of oxygen by hemoglobin can be illustrated in an oxygen dissociation curve. This curve demonstrates the proportion of oxygen saturation compared to the partial pressure of oxygen. At high partial pressures of oxygen, as found in the lungs, the hemoglobin becomes highly saturated with oxygen. As blood moves to areas of lower oxygen concentration, the hemoglobin releases oxygen.

Factors that affect oxygen binding

There are several factors that affect oxygen binding to hemoglobin:

  • Partial pressure of oxygen – Higher pO2 leads to more binding
  • Temperature – Lower temperatures favor more oxygen binding
  • pH – Lower pH (higher acidity) reduces oxygen binding
  • 2,3-Bisphosphoglycerate – This metabolite causes hemoglobin to release oxygen
  • Carbon dioxide – Higher levels cause oxygen to be released

Carbon dioxide transport

When carbon dioxide levels are high, some of it binds to hemoglobin and aminio acids in the blood. This allows carbon dioxide to be transported from tissues back to the lungs for expiration.

Fetal hemoglobin

During fetal development, the fetus has a different form of hemoglobin called fetal hemoglobin or hemoglobin F. It has a higher affinity for oxygen than adult hemoglobin, allowing the fetus to extract oxygen from the mother’s blood.

Conditions affecting hemoglobin

There are some medical conditions that affect hemoglobin and alter blood color:

  • Anemia – Low red blood cell count or hemoglobin levels can make blood appear more pale.
  • Jaundice – Buildup of bilirubin causes a yellowish tint.
  • Sulfhemoglobinemia – Caused by sulfur binding to hemoglobin, making blood appear blue.
  • Melanemia – Excess melanin causes blood to appear darker.

Other oxygen transport molecules

While humans and other vertebrates use hemoglobin, there are some invertebrates and bacteria that use other oxygen transport molecules:

Molecule Found in Color
Hemocyanin Mollusks, arthropods Blue
Chlorocruorin Annelid worms Green
Hemerythrin Marine invertebrates Pink

These other molecules contain copper or iron and bind oxygen reversibly like hemoglobin does. But they have different colors when oxygenated compared to the red of hemoglobin.

Heme synthesis

Heme is synthesized in a series of eight enzyme-catalyzed steps that occurs in the mitochondria and cytoplasm of the cell. The initial components are glycine and succinyl CoA derived from the Krebs cycle. The final step involves inserting iron into protoporphyrin IX to make heme.

Heme catabolism

When red blood cells reach the end of their lifespan, macrophages in the liver and spleen phagocytose them and break them down. An enzyme called heme oxygenase removes the iron from heme, creating biliverdin and carbon monoxide. Biliverdin is then converted to bilirubin, which gives a yellowish color if it accumulates.

Using heme as a cofactor

In addition to hemoglobin, heme is an important cofactor for other proteins in the body including:

  • Myoglobin – Oxygen storage in muscles
  • Cytochromes – Electron transport chain
  • Catalases and peroxidases – Hydrogen peroxide breakdown
  • Nitric oxide synthase – Affects vascular tone

The iron in heme facilitates oxygen binding, activation, and redox reactions in these physiologically important enzymes.

Heme regulation and disorders

Heme levels are tightly regulated by a feedback process because free heme is toxic. Disorders can occur if heme synthesis or breakdown is disrupted:

  • Porphyrias – Heme synthesis disorders causing neurologic dysfunction and photosensitivity
  • Anemias – Insufficient heme production
  • Hemochromatosis – Excess iron and heme absorption
  • Jaundice – Buildup of bilirubin from heme breakdown

Significance of red blood cell color

The red color of blood is vital for carrying out its oxygen transport function. The distinct bright red color of oxygenated blood allows us to monitor oxygenation clinically and spot dangerously low levels. If blood were another color, it would be much more difficult to evaluate oxygen delivery.

The color can indicate disease states like respiratory distress or carbon monoxide poisoning when blood appears darker. Blood color also provides visual cues for health conditions affecting heme and hemoglobin.


In summary, blood has a bright red color when oxygenated thanks to the heme group in hemoglobin. Heme contains iron that reversibly binds oxygen, allowing hemoglobin to deliver oxygen from the lungs to tissues. Other animals use different oxygen-binding molecules with various hues. Disrupting heme production can lead to anemia and other issues. So while blood could theoretically be another color, the red color of heme is evolutionarily advantageous for oxygen transport in vertebrates like humans.