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What is the dark red color of blood?

What is the dark red color of blood?

Blood is a vital bodily fluid that transports oxygen and nutrients throughout the body. The distinct dark red color of blood comes from the hemoglobin in red blood cells. Hemoglobin contains iron which binds to oxygen, giving blood its red hue. But why exactly is blood colored dark red instead of a brighter cherry red?

The Chemical Structure of Hemoglobin

Hemoglobin is composed of four protein subunits, each containing an iron atom bound to a heme group. It is the iron atom that gives hemoglobin the ability to bind oxygen. Iron can form six bonds – four within the heme group, one to histidine in the globin protein, and one to oxygen.

In its oxygenated form, iron takes on a red hue, lending a bright scarlet color to arteries carrying oxygen-rich blood from the lungs to the body. In contrast, deoxygenated blood traveling through veins back to the lungs appears darker, more of a maroon color. This is because some of the iron atoms have let go of their oxygen molecules, leaving fewer bonds to generate the vibrant red color.

Light Absorption of Hemoglobin

The precise shade of red blood depends on which wavelengths of light hemoglobin absorbs. Oxygenated hemoglobin (HbO2) has a brighter color because it absorbs more green light around 577 nm. Deoxygenated hemoglobin (Hb) appears darker, absorbing more blue light around 645 nm.

When blood is oxygen rich, the HbO2 predominates, selectively absorbing green wavelengths. This leaves mostly red wavelengths to be reflected back, giving bright scarlet blood its distinctive hue.

In deoxygenated blood, there is more Hb which absorbs more blue and purple light. With these wavelengths subtracted, blood appears a darker, more maroon red.

The Role of Plasma

While hemoglobin is responsible for the red color, the liquid plasma component of blood also impacts its appearance. Plasma makes up about 55% of total blood volume. It is mostly water (92% by volume) along with proteins, nutrients, waste products, and other solutes.

Because plasma is transparent yellow, it dilutes the vivid red hemoglobin color into a darker shade of red. Think of how adding white paint to bright red paint produces pink. Plasma has a similar effect in modulating the saturation of blood.

Blood Component Contribution to Color
Hemoglobin Gives blood its red hue
Plasma Dilutes and darkens the red color

Blood Oxygen Levels

The relative amounts of oxygenated and deoxygenated hemoglobin can vary in blood, affecting its exact shade from bright red to deep maroon. How do oxygen levels impact blood’s appearance?

Arterial blood has a scarlet color, containing about 97-98% oxygenated hemoglobin. As it releases oxygen to tissues, the blood becomes darker. In the veins, blood is about 75% oxygenated and 25% deoxygenated, giving it a deep red hue.

Certain medical conditions can drastically alter blood oxygenation. In carbon monoxide poisoning, carbon monoxide takes the place of oxygen, turning blood bright cherry red. Blood can also appear bluish with heart or lung dysfunction when oxygen levels severely drop.

Blood Type Oxygen Saturation Color
Arterial 97-98% Bright red
Venous 75% Dark red

Size of Blood Vessels

The size of the blood vessels also influences blood’s redshift. In the thick arteries and veins near the heart, blood has more volume and a vivid red appearance. But as vessels branch into tiny capillaries, the thinner blood takes on a more muted, brownish red tone. There are thousands of miles of capillaries in the body!

Think about food coloring – a few drops in water creates a deep vibrant hue, but that color becomes more diluted as more water is added. The same principle applies to blood in the circulatory system.

Individual Variation

While oxygenation state is the major factor impacting blood color, there are some individual differences as well. Skin tone affects perception – blood appears brighter against pale skin compared to darker skin. Blood nutrients like iron also cause variation. People with anemia have a reduced red blood cell count and associated paler blood.

There can also be dietary impacts. After eating antioxidants containing lycopene, found in foods like tomatoes, blood plasma takes on a redder tint. Carotenoid-containing foods like carrots may also introduce pigments that can subtly influence blood’s appearance.

Significance of Blood Color

The color of blood provides valuable medical insight. Bright red oxygenated blood indicates arteries and vital transport of oxygen. Dark red deoxygenated blood signifies veins carrying blood back for gas exchange. An abnormal color can signal issues with oxygen delivery or carbon monoxide poisoning.

Blood also varies in color depending on where in the body it has circulated. For example, intestinal blood will appear darker due to metabolic waste products absorbed from food. The breakdown of hemoglobin into bilirubin also causes a yellowish tint.

While we often think of blood as being red, its exact shade of crimson offers a window into the body’s intricate processes. The next time you get a cut or see blood drawn, take note of its precise color – it may just reveal something about your health!


In summary, blood has a dark red color due to the properties of hemoglobin, the relative amounts of oxygenated and deoxygenated blood, dilution by plasma, vessel size, and individual variation. While hemoglobin gives blood its distinct red hue, the exact shade can range from bright scarlet to deep maroon depending on oxygen levels. Changes in blood color beyond the normal spectrum often indicate an underlying issue that warrants medical assessment.