There are no instances of mammals with nucleated RBC. Mammalian erythrocytes or red blood cells, contain non-nucleated cells when they are mature. These cells must be able to pass through the tiniest of capillaries in order to transport oxygen to the body's cells. By shedding their nucleus during development, cells make extra space for the hemoglobin to fit inside the cell. Hemoglobin is an important transport protein, which contains the element iron. Its molecule contains four polypeptide chains-2 alpha and 2 beta chains in adults, folded into a specific shape. The red blood cells lose their other organelles including Golgi apparatus, mitochondria and endoplasmic reticulum. These cells are short-lived and the body makes 2.4 million new red blood cells in adult humans, per second. They develop in the bone marrow and live for 3-4 months.