Red blood cells in humans are a red color because it contains a lot of a protein called hemoglobin. When oxygen binds to hemoglobin, it makes it bright red. Your blood is oxygenated by your lungs as it passes through, which is why it is bright red leaving the lungs. Even after your blood deposits oxygen to your tissues and starts to return to your lungs, it still has oxygen bound to hemoglobin however there is less bound so your blood is a dark red. Blood in other animals such as squids is blue. This is because the protein that binds oxygen in their blood turns dark blue when oxygenated.
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