What is the name of the organic molecule that carries oxygen in the blood, and gives blood its red color?

Packed by the hundreds of millions into each red blood cell, this complex protein is the body's primary oxygen courier. Its name is a clue to its structure: 'heme' refers to an iron-containing compound, and 'globin' refers to the protein that houses it. The iron atom at the center of each heme group is what physically binds to oxygen in the lungs, and it is this same iron that gives blood its signature red hue. When oxygen is bound, the molecule reflects red light, making arterial blood a bright scarlet.

This molecule is a marvel of efficiency. It is composed of four protein chains, each holding its own heme group. This structure allows a single hemoglobin molecule to transport up to four oxygen molecules at once. As blood circulates, it picks up a full cargo of oxygen in the lungs to be delivered throughout the body. After releasing this oxygen to fuel the body's tissues, it darkens to a purplish-red for its return trip to the lungs.

While this iron-based system is essential for humans and other vertebrates, it's not the only solution in the animal kingdom. Some creatures, like the horseshoe crab, have evolved a different oxygen-carrying molecule called hemocyanin. Instead of iron, it uses copper, which results in a striking blue-colored blood.