Sunday, 22 March 2009

How many blood cells are produced daily by the bone marrow?

Although in children most of the blood cells are produced by the long bones, by the time adulthood beckons, they are largely produced by the marrow of the vertebrae, sternum and ribs.  The half-life of the various components is often quite short.  Half the platelets produced today will be gone in 4 days, and neutrophils only last about 8 hours! Furthermore, you require an enormously large number of cells in the blood stream.  Erythrocytes, for instance, are typically present in a concentration of 5 billion cells per millilitre.  (Think of that the next time you spill a drop of blood.)

The upshot of all this is that the marrow must be continually producing an enormous amount of new cells.  When you add it all up, the figure actually comes out as 100 billion cells per day.

By far the most numerous cell type is the red blood cell (erythrocyte), which outnumbers white blood cells by 700 to 1. Platelets have a concentration of 150 to 450 million per millilitre.  Overall, a typical thin smear of blood, once stained appropriately, looks something like this:

(a) points to two of the many red blood cells, (b) is a neutrophil, (c) is an eosinophil and (d) is a lymphocyte.  Notice how many red cells there are, compared with other cell types.  Those little dots in between the cells are mostly platelets.

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