Disorders affecting red cells
The quantity of red blood cells in normal human beings
varies with age and sex as well as with external conditions,
primarily atmospheric pressure. At sea level an average
normal manhas 5,400,000 red blood cells per cubic millimetre
of blood. These carry an average of 16 grams of hemoglobin
per 100 millilitres of blood. If such blood is centrifuged
so that the red cells are packed in a special tube known
as the hematocrit, they are found, on the average, to
occupy 47 percent of the volume of the blood. In the
average woman the normal figures are lower than this
(red cell count 4,800,000; hemoglobin 14 grams; volume
of packed red cells 42 percent). In the newborn infant
these values are higher but decrease in the course of
the first several weeks of postnatal life to levels
below those of the normal woman; thereafter, they rise
gradually. The differences in male and female blood
begin to appear at about the time of puberty. It should
be emphasized that these figures represent average values;
those found in normal persons range approximately 15
percent on either side of this mean.
From the physiological standpoint it is the quantity
of hemoglobin in the blood that is important because
this iron-containing protein is required for the transport
of oxygen from the lungs to the tissues. In disease,
as well as in certain situations in which physiological
adjustments take place, the quantity of hemoglobin may
be reduced below normal levels, a condition known as
anemia, or may be increased above normal, leading to
erythrocytosis (also called polycythemia).
