Normal
red blood cells are biconcave disc having a mean diameter of about 7.8
micrometers. The shapes of red blood cells can change remarkably as the cells
pass through capillaries. Actually, the red blood cell is a ‘bag’ that can be
deformed into almost any shape (Lee et al,
1993). Furthermore, because the normal cell has a great excess of cell membrane
for the quantity of material inside, deformation does not stretch the membrane
greatly and consequently
, does not rupture the cell as would be with many other
cells (Lee et al, 1993).
In the early few weeks of embryonic
life, primitive nucleated red blood cells are produced in the yolk sac. During
the middle trimester of gestation, the liver is the main organ for production
of red blood cells, although reasonable numbers of red blood cells are also
produced by the spleen and lymph nodes. Then, during the last mouth or so of
gestation and after birth, red blood cells are produced exclusively by the bone
marrow. (Lee et al,1993).
Figure
1 – Relative rates of RBC production in different ages.
(Guyton
et al, 1994).
As shown in figure 1, the bone marrow of
essentially all bones produces red blood cells until a person is 5years old
;but the marrow of the long bones, except for the proximal portions of the
humeri and tibiae, becomes quite fatty and produces no more red blood cells after
about age of 20 years. Beyond this age, most red blood cells are produced in
the marrow of the membranous bones, such as the vertebrae, sternum, ribs and
ilia. Even in these bones the marrow becomes less productive as age increases.
The total mass of red blood cells in
the circulating system is regulated within narrow limits so that an adequate
number of red cells is always available to provide sufficient tissue
oxygenation and yet so that the cells do not become so concentrated that they
impede blood flow.
Any
condition that causes the quantity of oxygen transported to the tissues to
decrease ordinarily increases the rate of red blood cell production (Guyton et al, 1996). Thus, when a person
becomes extremely anemic as a result of hemorrhage or another condition, the
bone marrow immediately began to produce large quantities of red blood cells.
Also destruction of major portions of the bone marrow by any means especially,
X – ray therapy, causes hyperplasia of the remaining bone marrow, thereby
attempting to supply the demand for red blood cells in the body. At a very high
altitudes, where the quantity of oxygen in the air is greatly decreased,
insufficient oxygen is transported to the tissues and red cell production is
considerably increased it is not the concentration of red blood cells in the
blood that controls the cells to transport oxygen to the tissues in relation to
the tissue, demand for oxygen.