Cardiovascular Physiology
427
occurs throughout the body but is most striking in rapidly
proliferating cells, including erythrocyte precursors. Thus,
fewer erythrocytes are produced when folic acid is defi cient.
The production of normal erythrocyte numbers also
requires extremely small quantities (one-millionth of a gram
per day) of a cobalt-containing molecule,
vitamin B
12
(also
called cobalamin), because this vitamin is required for the
action of folic acid. Vitamin B
12
is found only in animal prod-
ucts, and strictly vegetarian diets are defi cient in it. Also,
the absorption of vitamin B
12
from the gastrointestinal tract
requires a protein called
intrinsic factor,
which is secreted by
the stomach. Lack of this protein, therefore, causes vitamin B
12
defi ciency, and the resulting erythrocyte defi ciency is known
as
pernicious anemia
.
Regulation of Erythrocyte Production
In a typical person, the total volume of circulating erythrocytes
remains remarkably constant because of refl exes that regulate
the bone marrow’s production of these cells. In the previous
section, we stated that iron, folic acid, and vitamin B
12
must
be present for normal erythrocyte production, or
erythropoi-
esis.
However, none of these substances constitutes the signal
that regulates the production rate.
The direct control of erythropoiesis is exerted primarily
by a hormone called
erythropoietin,
which is secreted into
the blood mainly by a particular group of hormone-secreting
connective tissue cells in the kidneys (the liver also secretes
this hormone, but to a much lesser extent). Erythropoietin
acts on the bone marrow to stimulate the proliferation of
Storage (mainly in liver)
Dietary absorption
Loss (urine, skin cells,
sweat, menstrual
blood)
Spleen
(and liver)
Old erythrocyte
removal
New erythrocyte
formation
Erythrocyte
hemoglobin
All other cells
Blood
vessels
Plasma iron
3 mg
Iron recirculation
Iron recirculation
Bone
marrow
Figure 12–68
Summary of iron balance. The thickness of the arrows corresponds approximately to the amount of iron involved. In the steady state, the rate
of gastrointestinal iron absorption equals the rate of iron loss via urine, skin, and menstrual fl ow.
Adapted from Crosby.
previous page 455 Vander's Human Physiology The Mechanisms of Body Function read online next page 457 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off