Deoxygenated Blood Is Blue

Human blood is never blue. Whether oxygen-rich or oxygen-poor, blood is always some shade of red.

The misconception that deoxygenated blood is blue is widespread and often taught in basic biology classes, but it does not reflect how blood actually behaves inside the body.

What We Used to Think

Many people were taught that:

• Oxygenated blood is bright red
• Deoxygenated blood turns blue
• Veins appear blue because they carry blue blood

This idea was reinforced by textbook diagrams, simplified explanations, and phrases like “blue veins,” especially in childhood education.

When It Was Disproved

By the 1970s and 1980s, physiology and optical research had already made it clear that:

• Hemoglobin remains red even without oxygen
• Blood drawn from veins is dark red, not blue
• The blue appearance of veins is an optical illusion

Despite this, the simplified explanation persisted in classrooms and popular science materials for decades afterward.

Why It’s Wrong

• Hemoglobin chemistry: Oxygenated hemoglobin is bright red; deoxygenated hemoglobin is darker red, not blue.
• Optical effects: Skin absorbs longer (red) wavelengths of light and reflects shorter (blue) wavelengths, making veins appear bluish through the skin.
• Direct observation: During surgery or blood draws, venous blood is visibly dark red, not blue.
• True blue blood is rare: Some animals (like horseshoe crabs) have blue blood due to copper-based hemocyanin—but humans do not.

In short: veins look blue, but the blood inside them never is.