Which substance gives blood its red color?

• A. Red blood cells
• B. White blood cells
• C. Platelets
• D. Plasma

Answer: (A)

The red color of blood comes from the pigment hemoglobin inside red blood cells. Hemoglobin contains iron in its heme group; when it binds oxygen it forms oxyhemoglobin, which is bright red, and even when oxygen is released it remains a reddish color, though darker. Plasma is mostly clear and lacks pigments, while white blood cells and platelets are colorless components. So the substance that gives blood its red color is the red blood cells carrying hemoglobin.