Red blood cells are formed inside the bone marrow of our body from special stem cells known as hematopoietic stem cells. Now, once the red blood cells are formed, they are released into the blood plasma, into our blood circulation of our cardiovascular system. Now, once the red blood cells are fully formed, they are terminally differentiated and that means they will not divide, they will not undergo mitosis again during their lifetime. Now, the main function of red blood cells is to carry oxygen. So red blood cells contain many, many hemoglobin proteins inside the cytoplasm. And these hemoglobin proteins inside the red blood cells are responsible for picking up that oxygen inside the lungs and dropping, delivering that oxygen to the tissues of our body.