The viscosity of the blood,
, depends almost entirely on the blood con-
centration of red blood cells. In severe anemia, the
blood viscosity may fall to as low as 1.5 times that of
water rather than the normal value of about 3. This
decreases the resistance to blood flow in the periph-
eral blood vessels, so that far greater than normal
quantities of blood flow through the tissues and return
to the heart, thereby greatly increasing cardiac output.
Moreover, hypoxia resulting from diminished trans-
port of oxygen by the blood causes the peripheral
tissue blood vessels to dilate, allowing a further
increase in the return of blood to the heart and
increasing the cardiac output to a still higher level—
sometimes three to four times normal. Thus, one of the
major effects of anemia is greatly increased cardiac
output,as well as increased pumping workload on the
heart.
The increased cardiac output in anemia partially
offsets the reduced oxygen-carrying effect of the
anemia, because even though each unit quantity of
blood carries only small quantities of oxygen, the rate
of blood flow may be increased enough so that almost
normal quantities of oxygen are actually delivered to
the tissues. However, when a person with anemia
begins to exercise, the heart is not capable of pumping
much greater quantities of blood than it is already
pumping. Consequently, during exercise, which
greatly increases tissue demand for oxygen, extreme
tissue hypoxia results, and acute cardiac failure
ensues.