527
CHAPTER FOURTEEN
Blood
RECONNECT
To Chapter 7, Blood Cell Formation, pages 202–203.
In the red bone marrow, hematopoietic stem cells divide
and give rise to
erythroblasts
(e
˘-rith
ro-blastz) that can syn-
thesize 2 million to 3 million hemoglobin molecules per sec-
ond. The erythroblasts also divide and give rise to many new
cells. The nuclei of these newly formed cells soon shrink and
are extruded by being pinched off in thin coverings of cyto-
plasm and cell membrane. The resulting cells are erythro-
cytes. Some of these young red cells may contain a netlike
structure (reticulum) for a day or two. This network is the
remainder of the endoplasmic reticulum, and such cells are
called
reticulocytes
(re
˘-tik
u-lo-sitz). This is the stage that
exits the bone marrow to enter the blood. When the reticu-
lum degenerates, the cells are fully mature.
The average life span of a red blood cell is 120 days.
During that time, it travels through the body about 75,000
times. Many red blood cells are removed from the circulation
each day, yet the number in the circulating blood remains
relatively stable. These numbers suggest a homeostatic con-
trol of the rate of red blood cell production.
A
negative feedback mechanism
using the hormone
erythropoietin
(e-rith
ro-poi
e
˘-tin) (EPO) controls the rate
of red blood cell formation. In response to prolonged oxygen
deF
ciency, EPO is released, from the kidneys and to a lesser
extent from the liver. (In a fetus, the liver is the main site
of EPO production.) At high altitudes, for example, although
the percentage of oxygen in the air remains the same, the
atmospheric pressure decreases, reducing availability of oxy-
gen. The amount of oxygen delivered to the tissues initially
decreases. As
f
gure 14.6
shows, this drop in oxygen triggers
release of EPO, which travels via the blood to the red bone
marrow and stimulates increased erythrocyte production.
After a few days, many new red blood cells begin to
appear in the circulating blood. The increased rate of produc-
tion continues until the number of erythrocytes in the circula-
tion supplies sufF cient oxygen to tissues. When the availability
of oxygen returns to normal, EPO release decreases and the
rate of red blood cell production returns to normal.
RECONNECT
To Chapter 13, Using Hormones to Improve Athletic
Per±ormance, page 489.
Other conditions can lower oxygen levels and stimulate
EPO release. These include loss of blood, which decreases
the oxygen-carrying capacity of the cardiovascular system,
and chronic lung diseases, which decrease the respiratory
surface area available for gas exchange.
PRACTICE
8
What is the typical red blood cell count for an adult male? For an
adult female?
9
Where are red blood cells produced?
10
How does a red blood cell change as it matures?
11
How is red blood cell production controlled?
In
sickle cell disease,
a single DNA base mutation changes one amino
acid in the protein portion of hemoglobin, causing hemoglobin to
crystallize in a low oxygen environment. This bends the red blood
cells containing the hemoglobin into a sickle shape, which blocks
circulation in small vessels, causing excruciating joint pain and dam-
aging many organs. As the spleen works harder to recycle the too
short-lived red blood cells, infection becomes likely.
Children with sickle cell disease are typically diagnosed at
birth and receive antibiotics daily for years to prevent infection.
Hospitalization for blood transfusions may be necessary if the person
experiences painful sickling “crises” of blocked circulation.
A cancer drug called hydroxyurea is used to reactivate produc-
tion of a form of hemoglobin normally produced only in a fetus. The
fetal hemoglobin slows the crystallization of the sickle hemoglobin.
Sickling is delayed, which enables the red blood cells to reach the
lungs—where fresh oxygen restores the cells’ normal shapes. A bone
marrow transplant or umbilical cord stem cell transplant can com-
pletely cure sickle cell disease, but has a 15% risk of fatality. Thanks
to pain management, transfusions, drugs, and transplants, half of
all sickle cell patients now live beyond their ±
ftieth birthdays. In the
1960s a²
ected individuals rarely survived childhood.
PRACTICE
4
How do blood cells form?
5
Describe a red blood cell.
6
How does the biconcave shape of a red blood cell make possible
its function?
7
What is the function of hemoglobin?
Red Blood Cell Counts
The number of red blood cells in a microliter (µL or mcL or
1 mm
3
) of blood is called the
red blood cell count
(RBCC or
RCC). Although this number varies from time to time even
in healthy individuals, the typical range for adult males is
4,600,000–6,200,000 cells per microliter, and that for adult
females is 4,200,000–5,400,000 cells per microliter. ±or chil-
dren, the average range is 4,500,000–5,100,000 cells per microli-
ter. These values may vary slightly with the hospital, physician,
and type of equipment used to make blood cell counts. The
number of red blood cells generally increases after several days
following strenuous exercise or an increase in altitude.
An increasing number of circulating red blood cells
increases the blood’s
oxygen-carrying capacity,
so changes in
this number may affect health. ±or this reason, red blood cell
counts are routinely consulted to help diagnose and evaluate
the courses of various diseases.
Red Blood Cell Production and Its Control
Red blood cell formation (erythropoiesis) initially occurs in
the yolk sac, liver, and spleen. After birth, these cells are
produced almost exclusively by tissue lining the spaces in
bones, F
lled with red bone marrow.
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