Blood gets its red color from a protein called hemoglobin in red blood cells, which contains iron that turns red when it binds oxygen.

What gives blood its color?

  • Red blood cells are packed with hemoglobin, a protein that carries oxygen through the body.
  • Each hemoglobin molecule has “heme” groups with iron at the center; this iron–oxygen chemistry is what makes blood appear red.

Why shades of red differ

  • Oxygen-rich blood (in arteries, just leaving the lungs) looks bright red because more oxygen is bound to hemoglobin.
  • Oxygen-poor blood (in veins, returning to the heart) looks darker, sometimes brick- or maroon-red, but it is never actually blue.

Quick myth check

  • Veins can look blue through the skin because of how light is absorbed and scattered in tissue, not because the blood is blue.
  • In all normal conditions, human blood is always some shade of red due to iron-containing hemoglobin in red blood cells.

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