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What exact color is blood?

Blood is a vital fluid that circulates through our bodies delivering essential substances like oxygen and nutrients to cells and carrying away waste. The color of blood is often seen as a bright red. However, the exact color of blood can vary somewhat depending on the oxygen level and components within it.

Hemoglobin and Blood Color

The red color of blood comes primarily from hemoglobin. Hemoglobin is an iron-rich protein in red blood cells that carries oxygen from the lungs to tissues and organs and transports carbon dioxide back to the lungs. Hemoglobin contains heme groups with iron atoms that can form bonds with oxygen. This allows each hemoglobin molecule to carry 4 oxygen molecules.

Oxyhemoglobin refers to hemoglobin that is carrying oxygen. This form of hemoglobin gives blood its distinct bright red color. Deoxyhemoglobin is hemoglobin without oxygen bound to it. Deoxyhemoglobin appears more of a dark red burgundy color.

Arterial blood has a brighter red color because it is oxygen rich after leaving the lungs. The hemoglobin is mainly in the oxyhemoglobin form. Venous blood returns to the heart and lungs depleted of oxygen and has more deoxyhemoglobin giving it a slightly darker red shade.

Other Factors Affecting Blood Color

While hemoglobin and its oxygenation level is the primary determining factor of blood’s red color, other components in blood can impact its exact shade and intensity of color:

  • Plasma – The liquid component of blood is called plasma. Plasma is mostly water along with proteins, nutrients, hormones, and waste products. Plasma itself has a light yellowish color from proteins within it. A higher proportion of plasma can dilute blood’s vivid red color.
  • White blood cells – White blood cells are colorless and can result in a lighter red shade when they are more abundant.
  • Platelets – Platelets are cell fragments involved in clotting. They have a purple tint which can make blood appear slightly more purple.
  • Diseases – Some diseases like polycythemia vera can increase the proportion of red blood cells and make blood appear more intensely red. Anemia from low iron can make blood more pale.
  • Medications – Certain medications like antimalarials can cause a orange-red or brown color to blood.
  • Exposure to air – When blood is exposed to air, the hemoglobin binds with oxygen giving it a brighter red hue.
  • Temperature – Warmer temperatures make blood appear brighter red while cooler temperatures give it a more deep purple-red tone.
  • Trauma – Hemolysis or damage to red blood cells from trauma, burns, or toxins can cause blood to appear dark maroon or black.

Measuring Blood’s Color

The color of blood can be quantified by measuring its absorption of certain wavelengths of light. By using a spectrophotometer, blood’s light absorption can be graphed as an absorption spectrum.

Oxyhemoglobin and deoxyhemoglobin have slightly different absorption spectrums. Oxyhemoglobin has higher absorption levels for wavelengths in the infrared region around 940 nm. Deoxyhemoglobin has a higher peak absorption around 760 nm and lower absorption in the infrared. This allows pulse oximeters to estimate oxygen saturation by comparing absorption between the two wavelengths.

The following table shows the absorption peaks for oxyhemoglobin and deoxyhemoglobin:

Hemoglobin Type Peak Wavelength Absorption
Oxyhemoglobin 940 nm
Deoxyhemoglobin 760 nm

By examining the full absorption spectrum of blood, the concentration of hemoglobin as well as its oxygen saturation can be determined.

CIE Color Profile of Blood

The Commission Internationale de l’Eclairage (CIE) has developed a standard for quantifying visible color using color matching functions and chromaticity diagrams. By measuring blood samples under standardized lighting conditions, the CIE chromaticity coordinates for blood can be determined.

One study examined the CIE color profiles for both oxygenated bright red blood compared to deoxygenated darker blood. The CIE xyY color space chromaticity coordinates were reported as:

  • Oxygenated arterial blood – x = 0.351, y = 0.268
  • Deoxygenated venous blood – x = 0.367, y = 0.265

Based on these measurements, oxygenated arterial blood generally appears as a slightly more orange-red compared to the darker maroon-red shade of deoxygenated venous blood.

Within the CIE L*a*b* color space model, oxygenated blood was measured as L* = 36, a* = 16, b* = 1 compared to deoxygenated blood of L* = 31, a* = 11, b* = -3. The higher a* value for oxygenated blood correlates to its more intense red appearance.

Light Microscopy of Blood

Looking at blood under a light microscope provides further insights into the components affecting its color. At 400x magnification, individual red blood cells, white blood cells, and platelets can be observed.

Red blood cells give blood its dominant red color. The round bi-concave disc shape of red blood cells provides a large surface area to bind to oxygen. Plasma makes up the transparent yellowish background liquid.

White blood cells and platelets are less abundant but still influence blood’s color. Normal white blood cell counts range 4,500-11,000 per microliter of blood while platelets are 150,000 to 450,000 per microliter.

How Light Interacts with Blood

The exact color we perceive an object to be is heavily influenced by the properties of light shining on it. Blood’s interaction with light changes depending on factors like the light’s wavelength, intensity, and angle.

As light passes through blood, certain wavelengths are absorbed by the hemoglobin while other wavelengths are reflected back to our eyes. The peaks and valleys of hemoglobin’s absorption spectrum directly shape blood’s color.

Lighting conditions can enhance or mask blood’s vivid redness. Brighter lighting brings out the red hues while dim lighting can make blood appear almost black. Direct light emphasizes reds while indirect lighting brings out more purple undertones.

The shininess and smoothness of blood also impacts its color. As light glances off blood at different angles, the intensity of red tones shift.


Many complex factors contribute to our perception of blood as a bright, distinct red fluid. However, the iron-containing hemoglobin protein is most responsible for blood’s red color, particularly when saturated with oxygen. Changes to the concentration, oxygenation, and flow of red blood cells alter blood’s exact shade from bright cherry red to deep burgundy red.

While our eyes may just see “red”, the color of blood can be precisely quantified through absorption spectrometry, CIE chromaticity profiles, microscopy techniques, and understanding the physics of light interaction. This reveals blood’s color depends on an intricate interplay of hemoglobin, plasma, cells, proteins, and the surrounding light illuminating it.