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.
In capillaries, oxygen is released to be used by the body's cells. Ninety-seven percent of the oxygen carried by the blood from the lungs is carried through hemoglobin. The other 3 percent is dissolved in the plasma. Hemoglobin allows blood to move 30 to 100 times more oxygen than could possibly be dissolved in the plasma alone.
In the lungs, where the oxygen level is high, hemoglobin combines loosely with oxygen. The hemoglobin then easily releases that oxygen into the capillaries, where the oxygen level is low. In 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. In women, the normal concentration is 14 grams per deciliter of blood.