Is human blood actually blue?

Is Human Blood Actually Blue?

No, human blood is never blue—it is always red, though the shade varies depending on oxygen saturation. Blood appears red because hemoglobin, the oxygen-carrying protein in red blood cells, absorbs light in a way that reflects red wavelengths 1.

Why Blood Is Always Red

• Oxygenated blood (arterial) is bright red due to oxyhemoglobin, which binds oxygen in the lungs and delivers it to tissues 1.
• Deoxygenated blood (venous) is dark red or maroon, not blue, because deoxyhemoglobin has a different light absorption spectrum but still reflects red wavelengths 1.
• Normal human blood contains less than 1% methemoglobin, which is a form of hemoglobin with oxidized iron that cannot carry oxygen effectively 1.

Why Veins Appear Blue Through Skin

The common misconception that blood is blue likely stems from the appearance of veins through the skin:

• Veins appear blue or greenish due to optical properties of skin and light scattering, not because the blood inside them is blue 2.
• Light penetration and absorption by skin tissues at different wavelengths, combined with the depth and diameter of vessels, creates the visual perception of blue color 2.
• The scattering and absorption characteristics of skin preferentially allow blue wavelengths to be reflected back to the observer, while red wavelengths are absorbed more deeply 2.
• Red telangiectasias have higher oxygen saturation (5.9 kPa) compared to blue telangiectasias (5.11 kPa), but both contain red blood—the color difference is due to vessel depth, diameter, and overlying tissue properties 3.

When Blood Can Appear Abnormally Colored

Methemoglobinemia (Blue-Gray Discoloration of Skin)

• Methemoglobin levels >10% cause visible cyanosis with a blue, lavender, or slate-gray appearance of the skin, lips, and mucous membranes 1.
• The blood itself remains dark brownish-red, not blue, but the skin appears blue due to the inability of methemoglobin to carry oxygen 1.
• This cyanosis does not improve with supplemental oxygen, distinguishing it from hypoxemia-related cyanosis 1.
• Methemoglobin levels of 20-30% are typical in congenital methemoglobinemia, causing intense blue discoloration that is present from birth 1.

Methylene Blue Treatment Effects

• Methylene blue, used to treat methemoglobinemia, can cause greenish-blue discoloration of tissues when the colorless metabolite leucomethylene blue is oxidized upon exposure to atmospheric oxygen 4.
• This has been observed in autopsy findings of the brain and heart after methylene blue treatment for septic shock 4.
• Methylene blue can also cause greenish-blue urine and bluish discoloration of skin and mucosa as documented adverse effects 4.

Clinical Pitfalls to Avoid

• Do not confuse visible blue veins with blue blood—this is an optical illusion created by light interaction with skin 2.
• Cyanosis (blue skin) indicates inadequate oxygenation or abnormal hemoglobin, not that the blood has turned blue 1.
• Pulse oximetry can be misleadingly normal in methemoglobinemia despite severe cyanosis, as standard pulse oximetry cannot reliably detect dyshemoglobinemias 5.
• Methemoglobin levels should be measured directly using spectrophotometry when methemoglobinemia is suspected, rather than relying on pulse oximetry 1, 5.

Human blood is always red—the shade depends on oxygenation, but it never turns blue under any physiological or pathological condition.