Red Blood Cells (RBC's):
The red blood cell (erythrocyte) makes up 99.9% of the formed elements. They have a red color from hemoglobin pigment, and they transport oxygen and carbon dioxide throughout the body. Red blood cells do not have a nucleus. They are anaerobic meaning they don't use up the oxygen that they carry. These cells are very flexible, and they cannot replicate. Red blood cells last about 120 days since they squeeze into small areas, and bump into all kinds of dissolved elements and cells. New red blood cells are made in the bone marrow of long bones, and flat bones of the skull. Erythropoietin is a protein that triggers bone marrow to produce more red blood cells.
http://images.iop.org/objects/phw/news/15/6/7/red-blood-cells.jpg
http://images.iop.org/objects/phw/news/15/6/7/red-blood-cells.jpg