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The main function of red blood cells is the transport of oxygen from the lungs to the body's cells. Red blood cells contain a protein called hemoglobin that actually carries that oxygen.n each molecule of hemoglobin there are four iron atoms. Each iron atom binds with one molecule of oxygen. The iron in hemoglobin is what gives blood its red color. Thirty-three percent of a red blood cell is hemoglobin. The normal concentration of hemoglobin is 15.5 grams per deciliter of blood in men.
This allows the hemoglobin inside them  to bind oxygen from the air. The red blood cells then circulate  around the body. When they reach a body part that is low in oxygen,  it automatically drops off their hemoglobin. This way, red blood  cells supply the entire body with oxygen.