Blood is red because it contains iron, bound up in a ring-like chemical structure called porphyrin within haemoglobin – the protein responsible for carrying oxygen around the body. Haemoglobin is crammed into our red blood cells, making them red too.
Red blood cells, white blood cells and platelets are the key components of blood, floating in clear plasma, but the sheer volume of red blood cells gives blood a red colour overall. Oxygen-rich blood is a bright red, whereas deoxygenated blood is a darker brownish red. The veins in your wrists may appear blue but they are red too – the blue colour is the result of the way light travels through your skin.
While all vertebrates share the same colour blood, blue blood does exist – eg horseshoe crabs have no haemoglobin, having opted instead for haemocyanin, a copper-based protein.
Answered by Alexandra Cheung