Why is blood red?

Human blood gets its color from hemoglobin, a protein in red blood cells that contains four iron (Fe²⁺) atoms. Each iron atom can bind one oxygen molecule. When oxygenated (in arteries), hemoglobin absorbs blue-green wavelengths and reflects vivid red light at ~620–750 nm. Deoxygenated blood (in veins) absorbs more red light and appears darker crimson — but never blue. The "blue veins" myth arises because skin and fat tissue filter the light. A single red blood cell contains about 270 million hemoglobin molecules and makes a full circulation loop in about 20 seconds.