Blood is red because of iron in a protein that carries oxygen around.

The longer version

Haemoglobin gives blood its colour because of the iron atom held at the centre of each of its subunits. That iron binds oxygen loosely and reversibly, and the way its electrons absorb and reflect light shifts depending on whether oxygen is attached.

Oxygen-rich blood reflects a bright scarlet, while blood that has delivered its oxygen appears darker and more purple through the vessel wall. The protein is packed inside red cells, which are shaped like flattened discs to squeeze through narrow capillaries. Veins near the surface can look blue because of how skin scatters light, not because the blood is blue. Without the iron, the molecule could not carry oxygen at all.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.