|MadSci Network: Immunology|
The basic answer to your question is simple. In the bone marrow is a special type of cell called a stem cell. These stem cells are usually dividing to constantly make new cells. Some of the new cells are new stem cells (that continue to divide); but, many of them go on to become our red blood cells and white blood cells.
The body is constantly producing new red blood cells (and white ones as well) because red blood cells only live so long and are constantly dying. Its a pretty rough ride going back and forth through little tiny blood vessels many times a day and red blood cells eventually get worn out. They last an average of about three months.
Interesting related information:
Our bodies can control the rate of red blood cell production. If our tissues are not getting quite enough oxygen, the liver can sense this and tells the stem cells to make more red blood cells than usual. It does this by secreting a messenger protein (called erythropoetin) into the bloodstream. Bone marrow stem cells can sense this protein, and when they do, they decide more often than usual to become a red blood cell rather than a white blood cell. This is what happens when we travel to higher altitudes. It takes a little while for all those red cells to be made, so it takes a while to "get used to" higher altitudes.
If you have any more questions, please email me and I will try my best to answer them.
National Institutes of Health
Try the links in the MadSci Library for more information on Immunology.