Red blood cells carry oxygen and nutrients to the tissues in the body and take away carbon dioxide. They are also called erythrocytes. They get their red color from a pigment called hemoglobin, and it is the hemoglobin that actually carries the oxygen and carbon dioxide.
Red blood cells are made in the bone marrow. A red blood cell begins as a round cell with a round nucleus, and as this cell grows inside the bone marrow, it picks up hemoglobin. Finally, when the red blood cell is mature, it sheds the nucleus, leaves the marrow and enters the bloodstream. The lack of a nucleus is one thing that makes a red blood cell different from other mature cells. It has an indented center and is able to deform itself enough to squeeze through capillaries.
A red blood cell is active for about four months. After this time, it leaves the circulatory system and the body destroys it in the spleen, liver, bone marrow and blood vessels. To replace these destroyed cells, the body has to create about two million red blood cells every second.
Every cubic millimeter of a man's blood contains about 5.2 million red blood cells. The corresponding count for women is about 4.6 million.