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These changes nudge the neighboring chains into a different shape, making them bind oxygen more easily.

However, deep purple deoxygenated blood appears blue as it flows through our veins, especially in people with fair skin.

This is due to the way that different colors of light travel through skin: blue light is reflected in the surface layers of the skin, whereas red light penetrates more deeply.

Oxygen binds reversibly to these iron atoms and is transported through blood.

Each of the protein chains is similar in structure to myoglobin, the protein used to store oxygen in muscles and other tissues.

Oxygenated blood is bright red: when you are cut, the blood you see is brilliant red oxygenated blood.

Deoxygenated blood is deep purple: when you donate blood or give a blood sample at the doctor's office, it is drawn into a storage tube away from oxygen, so you can see this dark purple color.

Then, as blood circulates through the body, the oxygen level drops while that of carbon dioxide increases.

In this environment, hemoglobin releases its bound oxygen.

As soon as the first oxygen molecule drops off, the protein starts changing its shape.

This prompts the remaining three oxygens to be quickly released.

However, the need for matching blood type, the short life of stored blood, and the possibility of contamination are still major concerns.