Eddie Thornton, age 13, of Lake Charles, La., for his question:
WHERE IS BLOOD PRODUCED.?
Blood is the fluid organ circulating through the body in a system of arteries and veins. The blood, which carries into the body the substances necessary to life and carries out the waste products, is produced in the bone marrow.
Except for a few lymphocytes, all blood cells are produced in the red, pulpy tissue found in the webs of the "flat" bones such as ribs, pelvis and spike. Yellow marrow in the cylindrical bones of arms and legs is fat, but it can change to active red marrow when the body needs more than the normal number of blood cells.
The fatty marrow is a reserve organ, important because normal blood cells can be produced only in the marrow.
The blood in a human body contains 25 million million blood cells. In an average size, 150 pound person, the volume of blood is about five quarts.
The size of the blood volume (and the mass of blood cells) is maintained by a sensor feedback system. Red blood cells have a life span of about 100 to 120 days. At the end of this time they die of old age and are destroyed. A sew cell takes the place of each old one.
Thus, each day about one percent of our red cells live out their time and are replaced. The total mass of red cells in the bloodstream is slightly more than two quarts, which means that less than three quarters of an ounce of red cells are produced each day by the bone marrow.
To maintain proper levels, the body uses a sensor in the kidney corresponding to a thermostat. When the concentration of red cells falls below a critical level, this sensor recognizes the deficiency and sends a chemical signal the hormone erythropoietin through the bloodstream to the marrow, instructing it to manufacture red cells.
When the level of red cells rises above a critical level, the sensor ceases to secrete erythropoietin. Lacking a signal, the marrow's output of red cells subsides.
When more blood cells are needed in the body, the marrow is informed and increases output. The step up can be evoked by blood loss, excessive blood cell destruction, or by an increase requirement. For example, more white cells are needed during infections.
Normally, part of the marrow consists of fat cells. If increased production is required over a long period, fat storage tissue is converted into functional tissues to help with output.
In certain diseases, fat cells replace functional tissue, resulting in a productive organ too small to meet normal demand a condition called apiastic anemia. In other diseases, functional tissue is replaced by abnormal cells, scar tissue or cancer. Sometimes the marrow cells themselves become abnormal, as in leukemia.
The marrow contains it: proliferating cells in a delicate scaffolding of fibers and fine blood vessels. This micro circulation of the marrow is unique. Normal marrow cells cannot develop in the micro circulation of any other tissue. Disturbances of the scaffolding can disrupt normal proliferation of blood cells.