In April of 2018, the journal Science Reports published an article on hemoglobin and the likelihood of people with Type 2 diabetes developing diabetic retinopathy. There are about 500,000 molecules of hemoglobin per red blood cell. Each hemoglobin molecule, due to its iron content, carries two oxygen molecules from the lungs through to the rest of the body. Oxygen is what makes red blood cells red. When the hemoglobin in red blood cells gives up its oxygen molecules to the rest of the body, the red blood cells become blue. This is why blood in the veins is blue, and blood in the arteries is red.