Hemoglobin is a protein that exists in the red blood cells. Hemoglobin contains iron atoms in its chemical structure which are responsible for binding with oxygen or carbon dioxide. In its final structure, Hemoglobin contains 4 sites that can bind to oxygen. Despite that, it's difficult for a hemoglobin molecule to bind with four oxygen atoms at the same time because the structure will be complicated. Hemoglobin combined with oxygen is called oxyhemoglobin, while that combined to carbon dioxide is called carboxy hemoglobin.