Red Blood Cells (RBCs) are by far the most abundant cells in the blood. RBCs give blood its characteristic red color. In men, there are an average of 5,200,000 RBCs per cubic millimeter (microliter), and in women there are an average of 4,600,000 RBCs per cubic millimeter. 

There are several things about RBCs that make them unusual:

•  An RBC has a strange shape — a biconcave disc that is round and flat, sort of like a shallow bowl.
• An RBC has no nucleus. The nucleus is extruded from the cell as it matures.
• An RBC can change shape to an amazing extent, without breaking, as it squeezes single file through the capillaries. (Capillaries are minute blood vessels through which oxygen, nutrients and waste products are exchanged throughout the body.)
• An RBC contains hemoglobin, a molecule specially designed to hold oxygen and carry it to cells that need it. (The normal concentration of hemoglobin in blood is 15.5 grams per deciliter of blood in men, and 14 grams per deciliter of blood in women)
• The primary function of red blood cells is to transport oxygen from the lungs to the cells of the body.  
• Besides carrying oxygen to the cells of the body, the RBCs help to remove carbon dioxide (CO2) from the body.