Why is blood red?

Blood is red because of iron. Red blood cells are packed with a protein called haemoglobin, and each haemoglobin holds iron atoms. When iron grabs oxygen, it turns bright red — the same chemistry that turns an iron gate rusty-red.

That is why blood from a cut is bright red: it is full of oxygen. Blood in the veins, heading back to the lungs, has given up its oxygen and is a darker, duller red. It only looks blue through the skin because of the way light passes through flesh.

Not all animals have red blood. Octopuses use copper instead of iron, so their blood is blue. Some worms have green blood.

Haemoglobin is also why eating iron-rich food — spinach, drumstick leaves, dates, jaggery — matters. Too little iron means too little haemoglobin, and that is called anaemia.