Encyclopedia of Health and Medicine

A
aging, changes in the blood and lymph that
occur with The BLOOD undergoes a number of
normal changes across the span of the lifetime.
Blood cells have life spans ranging from a few
hours to decades. The blood continually renews
itself, producing millions of erythrocytes and
thousands of leukocytes every hour. Blood cell
production accelerates to meet unique health
needs, such as PREGNANCY or INFECTION.
The LIVER is the first organ in the developing
fetus to produce blood cells, primarily erythrocytes,
with supplemental production from the
SPLEEN and the THYMUS. At about five gestational
months the BONE MARROW has developed enough
to begin taking over blood cell production and by
birth is the primary structure for HEMATOPOIESIS.
Through childhood (until about age 16), nearly all
the bone marrow is red bone marrow that actively
produces blood cells. As the body matures yellow
bone marrow, a fibrous structure of connective tissue
and fat, gradually replaces the red bone marrow.
By adulthood only about 60 percent of the
bone marrow is red. This level remains fairly constant
until around age 70, when some red bone
marrow, primarily in the long bones, again transitions
to yellow marrow.
The bone marrow slows ERYTHROCYTE (red blood
cell) production in advanced age, putting fewer
erythrocytes into the blood’s circulation. The
reduced erythrocyte volume correspondingly
decreases the amount of available HEMOGLOBIN in
the blood, which diminishes the amount of oxygen
the blood can carry to the cells with each
heartbeat. This reduction commonly results in
decreased AEROBIC CAPACITY, showing lessened
ENDURANCE and longer recovery times with strenuous
physical activity, and may cause ANEMIA. The
spleen’s efficiency at removing old and defective
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erythrocytes from the circulation declines, an
accommodation that is somewhat a double-edged
sword. While this slowed hemolytic action allows
more erythrocytes to remain in the blood to
improve the blood’s capacity to carry oxygen, a
greater number of those erythrocytes are less
effective in this role.
The LYMPH structures and functions also change
with age. The lymphatic system becomes active at
about age two months when the child’s IMMUNE
SYSTEM begins to replace the protection from the
mother’s IMMUNITY. Most of the lymphocytes the
lymph tissues produce swarm to the THYMUS,
where they will come to maturity. The thymus
contains nearly the lifetime complement of immature
T-cell lymphocytes (called thymocytes in
immaturity) by about age 16, at which point the
thymus reaches its peak size and level of function.
As the T-cells reach maturity they leave the thymus
and migrate to other lymph structures
throughout the body where they reside until the
immune system needs them. As T-cell maturation
winds down the activity of the thymus decreases
and the thymus begins to shrink, diminishing by
early adulthood to a few clusters of lymph tissue.
After adulthood, the body can make only limited
additional T-cells. Health conditions that affect T-
cells, such as HIV/AIDS, that destroys them can
deplete the body’s supply of these vital protective
cells, depriving the body of its front line immune
defense.
With age lymphocyte production also decreases,
resulting in fewer circulating lymphocytes and a
corresponding reduced resistance to infection.
Later in life the spleen diminishes in size, ultimately
retreating to about half its size in early adulthood.
Its functional capacity decreases as well, resulting
in the spleen becoming less efficient at filtering