Hemoglobin is a complex protein molecule made up of four subunits of polypeptides, or globins, which are chains of amino acids. These globin subunits bind to non-ptorein heme groups containing an iron ion, which can bind to oxygen. These iron ions are in turn enclosed in rings called porphyrins.
Because hemoglobin contains iron, it's considered a metalloprotein. Four nitrogen atoms in the porphyin ring work with the iron ion. When the iron ion binds with oxygen, it becomes a deeper red color.
Hemoglobin has two proteins that allow it to carry and release oxygen. In adults, they are alpha globin and beta globin, but during pregnancy, gamma globin operates instead of beta globin in the fetal hemoglobin.
When red blood cells enter the alveoli, the hemoglobin within them combines with oxygen to form oxyhemoglobin. As the blood cells circulate through the rest of the body, the hemoglobin gives up the oxygen to the tissues. The globin part of hemoglobin takes up carbon dioxide, a waste material, and releases it back in the alveoli. Hemoglobin can carry oxygen and carbondioxide simultaneously.
Red blood cells are made up almost entirely of hemoglobin, but hemoglobin is also found in neurons, alveoli and white blood cells called macrophages.