RED BLOOD CELLS

Red cells, the heaviest particles in the blood, are tiny disc-shaped bodies, similar in appearance to small doughnuts without holes.  Each red cell has a diameter of seven to eight microns.  (A micron is a millionth of a meter.)  The body of an average adult contains approximately 25 trillion red cells.

The red cell begins its life in the bone marrow with a nucleus that directs the production of hemoglobin in the cell.  During red cell maturation, the nucleus is extruded.  When mature, the red cell leaves the bone marrow and enter the circulation where it lives for about 120 days.  There are red cells of all ages in the blood, ranging from those which have just left the bone marrow, to ones that are ready to be removed by the spleen and other members of the reticulo-endothelial system.

Hemoglobin, which gives the red cell its characteristic color, is a protein molecule compound of four subunits called globin surrounding two atoms of iron.  Hemoglobin has the unique property of combining with oxygen and giving it up to tissues, depending on the local oxygen tension.  Each red cell contains millions of these molecules, permitting rapid oxygen delivery and return of CO2 to the lungs.  These duties are performed countless times during the red cell's 120-day lifespan.
 

Click on each blood type to learn more:
Red Cells
White Cells
Platelets
Plasma