Why is blood red instead of blue?

Blood is red because of hemoglobin, the oxygen-carrying protein packed into red blood cells. At its core sits a heme group: an iron ion held inside a ring-shaped molecule called a porphyrin, built from four smaller rings joined together with the iron sitting right in the middle. That iron can grab a molecule of oxygen and let it go again, which is exactly what makes hemoglobin so good at ferrying oxygen through the body, and it's also what colors blood, since the iron's reaction with light changes depending on whether it's holding oxygen or not.

Oxygen-rich blood, the kind in arteries and capillaries, is bright red, because oxygen gives the iron-containing heme group a strong red color. Blood that has given up its oxygen, the kind flowing back through veins, is a darker shade of red, since hemoglobin absorbs light slightly differently once the oxygen is gone. Neither shade is blue: the veins you can see through your skin only look blue because of how skin scatters light and how your brain's visual system interprets that scattered light, not because the blood inside them is actually blue.

A few unusual situations change blood's red even further. Carbon monoxide poisoning turns blood an unnaturally bright red, because carbon monoxide latches onto hemoglobin and forms a compound called carboxyhemoglobin. Cyanide poisoning has the opposite problem: the body can't use the oxygen it's carrying, so venous blood stays oxygenated and even redder than normal. Rarer conditions can tip blood toward brown or a bluish dark red instead.

The Swedish physician Robin Fåhræus even suggested that the ancient idea of four 'humors' inside the body came from watching blood settle in a glass: left still for about an hour, it separates into a dark clot, a layer of red cells, a whitish layer of white cells, and clear yellow serum on top, matching the black bile, blood, phlegm, and yellow bile that ancient Greek medicine described.